Probability

1^st ball can go any of the 3 boxes. So total choices for 1^st ball = 3

2^nd ball can also go any of the 3 boxes. So total choices for 2^nd ball = 3
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12^th ball can go any of the 3 boxes. So total choices for 12^th ball = 3

Total choices for all 12 balls = $3 \times $$3 \times $$3 \times $.................12 times = 3¹².

Now question says choose 3 balls from 12 balls. So no of ways = ${}^{12}{C_3}$ ways.
And then put it in a box. No of ways we can put = ${}^{12}{C_3} \times 1$ ways.

Now we have 9 balls left and we have to put those 9 balls in the remaining 2 boxes.

Each ball can go to any of the 2 boxes, so for each ball there is 2 choices.

$\therefore$ Total ways for 9 balls = 2⁹

$\therefore$ Total ways we can put those 12 balls in the boxes = ${}^{12}{C_3} \times 1 \times {2^9}$

$\therefore$ Required probability = ${{{}^{12}{C_3} \times 1 \times {2^9}} \over {{3^{12}}}}$ = ${{55} \over 3}{\left( {{2 \over 3}} \right)^{11}}$

So option (C) is correct.

$P(\overline {A \cup B} ) = {1 \over 6}$

or, $1 - P(A \cup B) = {1 \over 6}$

$\therefore$ $P(A \cup B) = 1 - {1 \over 6} = {5 \over 6};$ $P(A \cap B) = {1 \over 4}$; and $P(\overline A ) = {1 \over 4};$

$\therefore$ $P(A) = 1 - P(\overline A ) = 1 - {1 \over 4} = {3 \over 4}$

We know, $P(A \cup B) = P(A) + P(B) - P(A \cap B)$

or, ${5 \over 6} = {3 \over 4} + P(B) - {1 \over 4}$

or, $P(B) = {5 \over 6} - {1 \over 2} = {1 \over 3}$

Now, $P(A)\,.\,P(B) = {3 \over 4}.\,{1 \over 3} = {1 \over 4} = P(A \cap B)$

i.e., events A and B are mutually independent.

Since the probability of A and B are different, so they are not equally likely events.

Therefore, (A) is the correct option.

Each question has 3 alternative and exactly one is correct.

$\therefore$ Probability of giving correct answer P(correct) = ${1 \over 3}$

$\therefore$ Probability of giving wrong answer P(wrong) = ${2 \over 3}$

Here student give 4 or more correct answer.

$\therefore$ Student give either 4 correct answer or 5 correct answer.

Using Binomial Distribution,

Required probability = ${}^5{C_4}{\left( {{1 \over 3}} \right)^4}{\left( {{2 \over 3}} \right)^1}$ + ${}^5{C_5}{\left( {{1 \over 3}} \right)^5}{\left( {{2 \over 3}} \right)^0}$ = ${{11} \over {{3^5}}}$

Given set S = $\left\{ {1,2,3,..8} \right\}$

Choosing 3 numbers from 8 numbers can be done ${{}^8{C_3}}$ ways.

Choosing 3 numbers from 8 numbers while minimum no is 3 can be done $1 \times {}^5{C_2}$ ways.

$\therefore$ Probablity P(min = 3) = ${{1 \times {}^5{C_2}} \over {{}^8{C_3}}}\,$

Choosing 3 numbers from 8 numbers while maximum no is 6 can be done $1 \times {}^5{C_2}$ ways.

$\therefore$ Probablity P(max = 6) = ${{1 \times {}^5{C_2}} \over {{}^8{C_3}}}\,$

Choosing 3 numbers from 8 numbers while minimum number 3 and maximum no is 6 can be done $1 \times {}^2{C_1} \times 1$ ways.

$\therefore$ $P\left( {\min = 3 \cap \max = 6} \right)$ = ${{1 \times {}^2{C_1} \times 1} \over {{}^8{C_3}}}$

The probability that their minimum is $3,$ given that their maximum is $6,$ is :
$P\left( {{{\min = 3} \over {\max = 6}}} \right)$

= ${{P\left( {\min = 3 \cap \max = 6} \right)} \over {P\left( {\max = 6} \right)}}$

= ${{{{{}^2{C_1}} \over {{}^8{C_3}}}} \over {{{{}^5{C_2}} \over {{}^8{C_3}}}}}$

= ${{{}^2{C_1}} \over {{}^5{C_2}}}$

= ${{1 \over 5}}$

Here is 5 trials.
So according to Bernoulli trial n = 5

P( at least one failure) = 1 - P( no failure)

According to question,
1 - P( no failure) $ \ge {{31} \over {32}}$

$ \Rightarrow $ 1 - P( x = 5 ) $ \ge {{31} \over {32}}$

[Note: no failure = all success]

$ \Rightarrow $ 1 - ${}^5{C_5}\,{p^5}$ $ \ge {{31} \over {32}}$

$ \Rightarrow $ 1 - ${p^5}$ $ \ge {{31} \over {32}}$

$ \Rightarrow $ ${p^5}$ $ \le $ ${1 \over {32}}$

$ \Rightarrow $ $p$ $ \le $ ${1 \over {2}}$

$\therefore$ $p$ $ \in $ $\left[ {0,{1 \over 2}} \right]$

Note: $p$ can not be less than 0 as probability is always $\ge$ 0 and $\le$ 1.

Given that $C \subset D$ means $C$ is present entirely inside $D$. Which is shown below.

$P\left( {{C \over D}} \right)$ = ${{P\left( {C \cap D} \right)} \over {P\left( D \right)}}$ = ${{P\left( C \right)} \over {P\left( D \right)}}$

As $C \cap D$ means common part of events C and D which is equal to C.

$0 \le P\left( D \right) \le 1$

$\therefore$ ${{P\left( C \right)} \over {P\left( D \right)}} \ge P\left( C \right)$

Note: Here we are dividing with ${P\left( D \right)}$ which is $ \le 1$ and $ \ge 0$, as we know on dividing with a number n in the range $0 \le n \le 1$ we get always more than or equal to the original number.

Out of nine balls three balls can be chosen = ${}^9{C_3}$ ways

$\therefore$ Sample space = ${}^9{C_3}$ = ${{9!} \over {3!6!}}$ = ${{9 \times 8 \times 7} \over 6}$ = 84

According to the question, all three ball should be different. So out of 3 red balls 1 is chosen and out of 4 blue 1 is chosen and out of 2 green 1 is chosen.

$\therefore$ Total cases = ${}^3{C_1} \times {}^4{C_1} \times {}^2{C_1}$ = 3 $ \times $ 4 $ \times $ 2 = 24

$\therefore$ Probability = ${{24} \over {84}}$ = ${{2} \over {7}}$

Four numbers can be chosen ${}^{20}{C_4}$ ways.

When common difference d = 1 then the possible sets are (1, 2, 3, 4) (2, 3, 4, 5) (3, 4, 5, 6) ................. (17, 18, 19, 20) = 17 sets
So when d = 1 then 17 different AP's are possible with 4 numbers.

Now let's create a table of all possible sets -

Common Difference (d)	Possible Sets	No of AP
d = 1	(1, 2, 3, 4) (2, 3, 4, 5) (3, 4, 5, 6) ................. (17, 18, 19, 20)	17
d = 2	(1, 3, 5, 7) (2, 4, 6, 8) (3, 5, 7, 9) ................. (14, 16, 18, 20)	14
d = 3	(1, 4, 7, 10) (2, 5, 8, 11) (3, 6, 9, 12) ................. (11, 14, 17, 20)	11
d = 4	(1, 5, 9, 13) (2, 6, 10, 14) (3, 7, 11, 15) ................. (8, 12, 16, 20)	8
d = 5	(1, 6, 11, 16) (2, 7, 12, 17) (3, 8, 13, 18) (4, 9, 14, 19) (5, 10, 15, 20)	5
d = 6	(1, 7, 13, 19) (2, 8, 14, 20)	2

$\therefore$ Total no of AP = 17 + 14 + 11 + 8 + 5 + 2 = 57

$\therefore$ Required probability = ${{57} \over {{}^{20}{C_4}}}$ = ${1 \over {85}}$

$\therefore$ Statement - 1: is true.

$\therefore$ Statement - 2: is false as common difference can also be $ \pm 6$.

Sample space = {00, 01, 02, 03, ..........49} = 50 tickets

n(S) = 50

n(Sum = 8) = { 08, 17, 26, 35, 44 } = 5

n(Product = 0) = { 00, 01, 02, 03, 04, 05, 06, 07, 08, 09, 10, 20, 30, 40 } = 14

$\therefore$ Probability when product is 0 = P(Product = 0) = ${14 \over {50}}$

n(Sum = 8 $ \cap $ Product = 0) = { 08 } = 1

$\therefore$ Probability when sum is 8 and product is 0 = P(Sum = 8 $ \cap $ Product = 0) = ${1 \over {50}}$

Required probability,
$P\left( {{{Sum = 8} \over {Product = 0}}} \right)$

= ${{P\left( {Sum = 8 \cap Product = 0} \right)} \over {P\left( {Product = 0} \right)}}$

= ${{{1 \over {50}}} \over {{{14} \over {50}}}}$

=${1 \over {14}}$

$\therefore$ Option (D) is correct.

Given that, no of trials = n
Probability of success (p) = ${{1 \over 4}}$
$\therefore$ Probability of no success = 1 - ${{1 \over 4}}$ = ${{3 \over 4}}$

As we know, probability of at least one success = 1 - probability of no success

$\therefore$ According to the question,

1 - (Probability of no success in n trials) $\ge$ ${9 \over {10}}$

$ \Rightarrow $ 1 - P(x = 0) $\ge$ ${9 \over {10}}$

$ \Rightarrow $ 1 - ${}^n{C_0}$${\left( {{1 \over 4}} \right)^0}$${\left( {{3 \over 4}} \right)^n}$ $\ge$ ${9 \over {10}}$

$ \Rightarrow $ 1 - ${\left( {{3 \over 4}} \right)^n}$ $\ge$ ${9 \over {10}}$

$ \Rightarrow $ ${\left( {{3 \over 4}} \right)^n}$ $\le$ ${1 \over {10}}$

$ \Rightarrow $ ${\left( {{4 \over 3}} \right)^n}$ $\ge$ $10$

[Taking log on both sides]

$ \Rightarrow $ $n\left( {\log _{10}^4 - \log _{10}^3} \right)$ $\ge$ ${\log _{10}^{10}}$

$ \Rightarrow $ $n\left( {\log _{10}^4 - \log _{10}^3} \right)$ $\ge$ $1$

$ \Rightarrow $ $n$ $\ge$ ${1 \over {\left( {\log _{10}^4 - \log _{10}^3} \right)}}$

$\therefore$ Option (D) is correct.

No of outcome for a die = { 1, 2, 3, 4, 5, 6 }
According to the question,

A = { 4, 5, 6 }

$\therefore$ P(A) = ${3 \over 6}$

B = { 1, 2, 3, 4 }

$\therefore$ P(A) = ${4 \over 6}$

A $ \cap $ B = { 4 }

So P(A $ \cap $ B) = ${1 \over 6}$

We know, $P\left( {A \cup B} \right)$ = $P\left( A \right)$ + $P\left( B \right)$ - $P\left( {A \cap B} \right)$

$\therefore$ $P\left( {A \cup B} \right)$ = ${3 \over 6}$ + ${4 \over 6}$ - ${1 \over 6}$ = ${{7 - 1} \over 6}$ = ${6 \over 6}$ = 1

$\therefore$ Option (C) is correct.

Given that,

$P\left( {{A \over B}} \right) = {1 \over 2}$

$ \Rightarrow $ ${{P\left( {A \cap B} \right)} \over {P\left( B \right)}}$ = ${1 \over 2}$.............. equation (1)

$P\left( {{B \over A}} \right) = {2 \over 3}$

$ \Rightarrow $ ${{P\left( {A \cap B} \right)} \over {P\left( A \right)}}$ = ${2 \over 3}$.............. equation (2)

Dividing equation (1) by equation (2) we get,

${{P\left( A \right)} \over {P\left( B \right)}}$ = ${3 \over 4}$

$ \Rightarrow $ ${P\left( B \right)}$ = ${4 \over 3}$ $ \times $ ${P\left( A \right)}$
                   = ${4 \over 3}$ $ \times $ ${1 \over 4}$
                   = ${1 \over 3}$

$\therefore$ Option (B) is correct.

Probability of getting score 9 in a single throw

$ = {4 \over {36}} = {1 \over 9}.$

Probability of getting score 9 exactly twice

$ = {}^3{C_2} \times {\left( {{1 \over 9}} \right)^2} \times {8 \over 9} = {8 \over {243}}.$

The desired probability

= (0.7) (0.2) + (0.7) (0.8) (0.7) (0.2) + (0.7) (0.8) (0.7) (0.8) (0.7) (0.2) + .......

= 0.14 [1 + (0.56) + (0.56)² + .......]

= 0.14 $\left( {{1 \over {1 - 0.56}}} \right) = {{0.14} \over {0.44}} = {7 \over {22}}$ = 0.32

Required probability

$ = P(X = 0) + P(X = 1)$

$ = {{{e^{ - 5}}} \over {0!}}{5^0} + {{{e^{ - 5}}} \over {1!}}{5^1}$

$ = {e^{ - 5}} + 5{e^{ - 5}}$

$ = {6 \over {{e^5}}}$

Person 1st has three options to apply.

Similarly, person 2nd has three options t apply

and person 3rd has three options to apply.

Total cases = 3³

Now, favourable cases = 3 (An either all has applied for house 1 or 2 or 3)

So, probability $ = {3 \over {{3^3}}} = {1 \over 9}$.

Given that,

$P(\overline {A \cup B} ) = {1 \over 6}$, $P(A \cap B) = {1 \over 4}$, $P(\overline A ) = {1 \over 4}$

$\because$ $P(\overline {A \cup B} ) = {1 \over 6}$

$ \Rightarrow 1 - P(A \cup B) = {1 \over 6}$

$ \Rightarrow 1 - P(A) - P(B) + P(A \cap B) = {1 \over 6}$

$ \Rightarrow P(\overline A ) - P(B) + {1 \over 4} = {1 \over 6}$

$ \Rightarrow P(B) = {1 \over 4} + {1 \over 4} - {1 \over 6}$

$ \Rightarrow P(B) = {1 \over 3}$ and $P(A) = {3 \over 4}$

Clearly, $P(A \cap B) = P(A)P(B)$,

so, the events A and B are independent events but not equally likely.

In a position distribution,

$P(X = r) = {{{e^{ - \lambda }}{\lambda ^r}} \over {r!}}$ ($\lambda$ = mean).

Now, $P(X = r > 1.5) = P(2) + P(3) + $ ..... $\infty$

$ = 1 - \{ P(0) + P(1)\} $

$ = 1 - \left( {{e^{ - 2}} + {{{e^{ - 2}} \times 2} \over 1}} \right) = 1 - {3 \over {{e^2}}}$

The probability of speaking truth by A, P(A) = ${4 \over 5}$. The probability of not speaking truth by A, P($\overline A $) = 1 $-$ ${4 \over 5} = {1 \over 5}$.

The probability of speaking truth by B, P(B) = ${3 \over 4}$. The probability of not speaking truth of B, P($\overline B $) $ = {1 \over 4}$.

The probability that they contradict each other

$ = P(A) \times P(\overline B ) + P(\overline A ) \times P(B)$

$ = {4 \over 5} \times {1 \over 4} + {1 \over 5} \times {3 \over 4}$

$ = {1 \over 5} + {3 \over {20}} = {7 \over {20}}$

Given that mean = 4 = np = 4 and variance = 2

npq = 2 $\Rightarrow$ 4q = 2

$ \Rightarrow q = {1 \over 2}$

$\therefore$ $p = 1 - q = 1 - {1 \over 2} = {1 \over 2}$

Also, n = 8

Probability of 2 successes $ = P(X = 2) = {}^8{C_2}{p^2}{q^6}$

$ = {{8!} \over {2! \times 6!}} \times {\left( {{1 \over 2}} \right)^2} \times {\left( {{1 \over 2}} \right)^6} = 28 \times {1 \over {{2^8}}}$

$ = {{28} \over {256}}$

X : Mr. A selected wining horse

$\overline X $ : Mr. A did not select wining horse

Mr. A selected two horses, now probability of not wining the first horse which Mr. A choses = ${4 \over 5}$

And probability of not wining the second horse also which Mr. A choses = ${3 \over 4}$ (Here Mr. A out of remaining 4 horses choses one horse among 3 horses which did not win)

$\therefore$ $P(\overline X ) = {4 \over 5} \times {3 \over 4}$

$\therefore$ $P(X) = 1 - P(\overline X )$

$ = 1 - {4 \over 5} \times {3 \over 4}$

$ = 1 - {3 \over 5}$

$ = {2 \over 5}$

Mean = $np$ = 4
Variance = $npq$ = 2

$ \Rightarrow 4 \times q$ = 2
$ \Rightarrow$ $q$ = ${1 \over 2}$
p = 1 - q = 1 - ${1 \over 2}$ = ${1 \over 2}$
n = 8

Now P(X = 1) = ${}^8{C_1}{\left( {{1 \over 2}} \right)^1}{\left( {{1 \over 2}} \right)^{8 - 1}}$
                        = $8 \times {1 \over {{2^8}}}$
                        = ${1 \over {32}}$

Given $P\left( A \right) = {{3x + 1} \over 3},$ $P\left( B \right) = {{1 - x} \over 4}$ and $P\left( C \right) = {{1 - 2x} \over 2}$

We know for any event X, $0 \le P\left( X \right) \le 1$

$\therefore$ $0 \le {{3x + 1} \over 3} \le 1$
$ \Rightarrow - 1 \le 3x \le 2$
$ \Rightarrow - {1 \over 3} \le x \le {2 \over 3}$

$0 \le {{1 - x} \over 4} \le 1$
$ \Rightarrow - 3 \le x \le 1$

$0 \le {{1 - 2x} \over 2} \le 1$
$ \Rightarrow - 1 \le 2x \le 1$
$ \Rightarrow - {1 \over 2} \le x \le {1 \over 2}$

In the question given that A, B and C are mutually exclusive
So $P\left( {A \cup B \cup C} \right)$ = $P\left( {A} \right)$ + $P\left( {B} \right)$ + $P\left( {C} \right)$
 
$ \Rightarrow P\left( {A \cup B \cup C} \right)$ = ${{3x + 1} \over 3}$ + ${{1 - x} \over 4}$ + ${{1 - 2x} \over 2}$

$\therefore$ 0 $ \le $ ${{3x + 1} \over 3}$ + ${{1 - x} \over 4}$ + ${{1 - 2x} \over 2}$ $ \le $ 1

0 $ \le $ 13 - 3$x$ $ \le $ 12

$ \Rightarrow {1 \over 3} \le x \le {{13} \over 3}$

From all those relations, we get

$\max \left\{ { - {1 \over 3}, - 3, - {1 \over 2},{1 \over 3}} \right\}$ $ \le $ $x$ $ \le $ $\min \left\{ {{2 \over 3},1,{1 \over 2},{{13} \over 3}} \right\}$

So, ${1 \over 3} \le x \le {1 \over 2}$

$ \Rightarrow $ $ \Rightarrow x \in \left[ {{1 \over 3},{1 \over 2}} \right]$

Given $P\left( A \right) = {1 \over 2}$, $P\left( B \right) = {1 \over 3}$, $P\left( C \right) = {1 \over 4}$

So, $P\left( {\overline A } \right) = {1 \over 2}$ (Probablity that the problem can't be solve by A)
$P\left( {\overline B } \right) = {2 \over 3}$ (Probablity that the problem can't be solve by B)
and $P\left( {\overline C } \right) = {3 \over 4}$ (Probablity that the problem can't be solve by C)

Now the probablity that the problem is solved by any one student of A, B and C = 1 - the probablity that the problem is solved by none of the students of A, B and C

$P\left( {A \cup B \cup C} \right)$ = 1 - $P\left( {\overline A } \right)P\left( {\overline B } \right)P\left( {\overline C } \right)$

$ = 1 - {1 \over 2} \times {2 \over 3} \times {3 \over 4}$
$=$ ${3 \over 4}$

Total no of trials = 5
$\therefore$ n = 5
Odd no possibe are = {1, 3, 5} =3
Sample space = {1, 2, 3, 4, 5, 6} = 6

Probablity of getting odd number = ${3 \over 6}$ = ${1 \over 2}$
$\therefore$ p = ${1 \over 2}$

Formula for variance = npq
where q = 1 - p = 1 - ${1 \over 2}$ = ${1 \over 2}$
$\therefore$ Variance = $5 \times {1 \over 2} \times {1 \over 2}$ = ${5 \over 4}$

Given $P\left( {A \cup B} \right) = 3/4$,
$P\left( {A \cap B} \right) = 1/4,$
$P\left( {\overline A } \right) = 2/3$

We know, $P\left( A \right)$ = 1 - $P\left( {\overline A } \right)$
$\therefore$ $P\left( A \right)$ = 1 - ${2 \over 3}$ = ${1 \over 3}$

We know $P\left( {A \cup B} \right)$ = $P\left( A \right)$ + $P\left( B \right)$ - $P\left( {A \cap B} \right)$

$ \Rightarrow $${3 \over 4}$ = ${1 \over 3}$ + $P\left( B \right)$ - ${1 \over 3}$

$ \Rightarrow $ 1 = ${1 \over 3}$ + $P\left( B \right)$

$ \Rightarrow $ $P\left( B \right)$ = ${2 \over 3}$

We know $P\left( {\overline A \cap B} \right)$ = $P\left( B \right)$ - $P\left( {A \cap B} \right)$

So $P\left( {\overline A \cap B} \right)$ = ${2 \over 3}$ - ${1 \over4}$ = ${5 \over 12}$