ExamGOAL
Books
14
Subjective

If the letters of the word 'ASSASSINATION' are arranged at random. Find the probability that

(i) four S's come consecutively in the word.

(ii) two I's and two N's come together.

(iii) all A's are not coming together.

(iv) no two A's are coming together.

Explanation

Total number of letters in the word 'ASSASSINATION' are 13.

Out of which 3A's, 4S's, 2 I's, 2 N's, 1 T's and 10.

(i) If four S's come consecutively in the word, then we considers these 4 S's as 1 group. Now, the number of laters is 10 .

S S S S A A A I I N N T O
1 9

Number of words when all S's are together $=\frac{10!}{3!2!2!}$

Total number of word using letters of the word 'ASSASSINATION'

$$\begin{aligned} \quad & =\frac{13!}{3!4!2!2!} \\ \therefore \quad \text { Required probability } & =\frac{10!}{\frac{3!2!2!\times 13!}{3!4!2!2!}} \\ & =\frac{10!\times 4!}{13!}=\frac{4!}{13 \times 12 \times 11}=\frac{24}{1716}=\frac{2}{143} \end{aligned}$$

(ii) If 2 I's and 2 N 's come together, then there as 10 alphabets.

Number of word when 2 l's and 2 N's are come together

$$\begin{aligned} & =\frac{10!}{3!4!} \times \frac{4!}{2!2!} \\ \therefore \quad \text { Required probability } & =\frac{\frac{10!4!}{3!4!2!2!}}{\frac{13!}{3!4!2!2!}}=\frac{4!10!}{2!2!3!4!} \times \frac{3!4!2!2!}{13!} \\ & =\frac{4!10!}{13!}=\frac{4!}{13 \times 12 \times 11}=\frac{24}{13 \times 12 \times 11}=\frac{2}{143} \end{aligned}$$

(iii) If all A's are coming together, then there are 11 alphabets.

Number of words when all A's come together

$$=\frac{11!}{4!2!2!}$$

Probability when all A's come together

$$=\frac{\frac{11!}{4!2!2!}}{\frac{13!}{4!3!2!2!}}=\frac{11!}{4!2!2!} \times \frac{4!3!2!2!}{13!}=\frac{11!\times 3!}{13!}=\frac{6}{13 \times 12}=\frac{1}{26}$$

Required probability when all A's does not come together

$$=1-\frac{1}{26}=\frac{25}{26}$$

(iv) If no two A's are together, then first we arrange the alphabets except A's.

S S S S I N T I O N

All the alphabets except A's are arranged in $\frac{10!}{4!2!2!}$.

There are 11 vacant places between these alphabets.

So, 3 A's can be place in 11 places in ${ }^{11} C_3$ ways $=\frac{11!}{3!8!}$

$\therefore$ Total number of words when no two A's together

$$\begin{aligned} & =\frac{11!}{3!8!} \times \frac{10!}{4!2!2!} \\ \text { Required probability } & =\frac{11!\times 10!}{3!8!4!2!2!} \times \frac{4!3!2!2!}{13!}=\frac{10!}{8!\times 13 \times 12} \\ & =\frac{10 \times 9}{13 \times 12}=\frac{90}{156}=\frac{15}{26} \end{aligned}$$

15
Subjective

If a card is drawn from a deck of 52 cards, then find the probability of getting a king or a heart or a red card.

Explanation

$\because$ Number of possible event $=52$

and favourable events $=4$ king +13 heart +26 red $-13-2=28$

$\therefore \quad$ Required probability $=\frac{28}{52}=\frac{7}{13}$

16
Subjective

A sample space consists of 9 elementary outcomes $E_1, E_2, \ldots, E_9$ whose probabilities are

$$\begin{aligned} & P\left(E_1\right)=P\left(E_2\right)=0.08, P\left(E_3\right)=P\left(E_4\right)=P\left(E_5\right)=0.1 \\ & P\left(E_6\right)=P\left(E_7\right)=0.2, P\left(E_8\right)=P\left(E_9\right)=0.07 \end{aligned}$$

Suppose $$A=\left\{E_1, E_5, E_8\right\}, B=\left\{E_2, E_5, E_8, E_9\right\}$$

(i) Calculate $P(A), P(B)$ and $P(A \cap B)$.

(ii) Using the addition law of probability, calculate $P(A \cup B)$.

(iii) List the composition of the event $A \cup B$ and calculate $P(A \cup B)$ by adding the probabilities of the elementary outcomes.

(iv) Calculate $P(\bar{B})$ from $P(B)$, also calculate $P(\bar{B})$ directly from the elementary outcomes of $\bar{B}$.

Explanation

Given,

$$\begin{aligned} S & =\left\{E_1, E_2, E_3, E_4, E_5, E_6, E_7, E_8, E_9\right\} \\ A & =\left\{E_1, E_5, E_8\right\}, B=\left\{E_2, E_5, E_8, E_9\right\} \\ P\left(E_1\right) & =P\left(E_2\right)=0.08 \\ P\left(E_3\right) & =P\left(E_{4)}=P\left(E_5\right)=0.1\right. \\ P\left(E_6\right) & =P\left(E_7\right)=2, P\left(E_8\right)=P\left(E_9\right)=0.07 \end{aligned}$$

(i) $$\begin{aligned} P(A) & =P\left(E_1\right)+P\left(E_5\right)+P\left(E_8\right) \\ & =0.08+0.1+0.07=0.25 \end{aligned}$$

(ii)$$\begin{aligned} & P(A \cup B)=P(A)+P(B)-P(A \cap B) \\ & \begin{aligned} \text { Now, } P(B) & =P\left(E_2\right)+P\left(E_5\right)+P\left(E_8\right)+P\left(E_9\right) \\ & =0.08+0.1+0.07+0.07=0.32 \\ A \cap B & =\left\{E_5, E_8\right\} \\ P(A \cap B) & =P\left(E_5\right)+P\left(E_8\right)=0.1+0.7=0.17 \end{aligned} \end{aligned}$$

On substituting these values in Eq.(i), we get

$$P(A \cup B)=0.25+0.32-0.17=0.40$$

(iii) $A \cup B=\left\{E_1, E_2, E_5, E_8, E_9\right\}$

$$\begin{aligned} P(A \cup B) & =P\left(E_1\right)+P\left(E_2\right)+P\left(E_5\right)+P\left(E_8\right)+P\left(E_9\right) \\ & =0.08+0.08+0.1+0.07+0.07=0.40 \end{aligned}$$

(iv) $$\begin{aligned} & \because P(\bar{B})=1-P(B)=1-0.32=0.68 \\ & \text { and } \quad \bar{B}=\left\{E_1, E_3, E_4, E_6, E_7\right\} \\ & \therefore \quad P(\bar{B})=P\left(E_1\right)+P\left(E_3\right)+P\left(E_4\right)+P\left(E_6\right)+P\left(E_7\right) \\ & =0.08+0.1+0.1+0.2+0.2=0.68 \end{aligned}$$

17
Subjective

Determine the probability $p$, for each of the following events.

(i) An odd number appears in a single toss of a fair die.

(ii) Atleast one head appears in two tosses of a fair coin.

(iii) A king, 9 of hearts or 3 of spades appears in drawing a single card from a well shuffled ordinary deck of 52 cards.

(iv) The sum of 6 appears in a single toss of a pair of fair dice.

Explanation

(i) When a die is throw the possible outcomes are

$$\begin{aligned} S & =\{1,2,3,4,5,6\} \text { out of which } 1,3,5 \text { are odd, } \\ \therefore \text { Required probability } & =\frac{3}{6}=\frac{1}{2} \end{aligned}$$

(ii) When a fair coin is tossed two times the sample space is

$$S=\{H H, H T, T H, T T\}$$

In at least one head favourable enonts are $H H, H T, T H$

$\therefore$ Required probability $=\frac{3}{4}$

(iii) Total cards $=52$

$$\begin{aligned} \text { Favourable } & =4 \text { king }+2 \text { of heart }+3 \text { of spade }=4+1+1=6 \\ \therefore \quad \text { Required probability } & =\frac{6}{52}=\frac{3}{26} \end{aligned}$$

(iv) When a pair of dice is rolled total sample parts are 36 . Out of which $(1,5),(5,1),(2,4)$, $(4,2)$ and $(3,3)$.

$\therefore$ Required probability $=\frac{5}{36}$

18
MCQ (Single Correct Answer)

In a non-leap year, the probability of having 53 Tuesday or 53 Wednesday is

A
$\frac{1}{7}$
B
$\frac{2}{7}$
C
$\frac{3}{7}$
D
None of these