Probability

29. Total number of balls $=18$

Number of red balls $=8$

Number of black balls $=10$

i. Probability of getting red ball at first draw $=\frac{8}{18}=\frac{4}{9}$

The red ball is replaced after the first draw

$\therefore$ Probability of getting red ball in second draw $=\frac{8}{18}=\frac{4}{9}$

$\therefore$ Probability of getting both the balls red $=\frac{4}{9}\times \frac{4}{9}=\frac{16}{81}$

ii. Probability o

28. The sample space of given condition is

$S=\{1,2,3,4,5,6\}$

Let,  $P(A)=$ probability of getting an odd number in first throw

$\Rightarrow P(A)=\frac{3}{6}=\frac{1}{2}$

$P(B)=$ probability of getting an even number

$\Rightarrow P(B)=\frac{3}{6}=\frac{1}{2}$

Probability of getting an even number in three times $=\frac{1}{2}\times \frac{1}{2}\times \frac{1}{2}=\frac{1}{8}$

So, Probability of getting an odd number at least once

$=1-$ probability of getting an odd

27. Given,

$P(A)=0.3$

$P(B)=0.6$

$\mathrm{i.\; A}$ and $B$ are independent

$\Rightarrow P(A\cap B)=P(A).P(B)$

$=0.3\times 0.6$

$=0.18$

$\mathrm{ii.\; P}$ ( $A$ and not $B$ )

$\Rightarrow P(A\cap B^{\prime })=P(A)-P(A\cap B)$

$=0.3-0.18$

$=0.12$

$\mathrm{iii.\; P}$ ( $A$ or $B$ )

$\Rightarrow P(A\cup B)=P(A)+P(B)-P(A\cap B)$

$=0.3+0.6-0.18$

$=0.72$

$\mathrm{iv.\; P}$ (neither $A$ nor $B$ )

$\Rightarrow P(A^{\prime }\cap B^{\prime })=P(A\cup B{)}^{\prime }$

$=1-P(A\cup B)$

$=1-0.72$

$=0.28$

26. Given,

$P(A)=\frac{1}{2}$

$P(B)=\frac{7}{12}$

$P$ (not $A$ or not $B$ )= $\frac{1}{4}$

$\Rightarrow P(A^{\prime }\cup B^{\prime })=P(A\cap B{)}^{\prime }$

$\Rightarrow P(A\cap B{)}^{\prime }=\frac{1}{4}$

$\Rightarrow 1-P(A\cap B)=\frac{1}{4}$

$\Rightarrow P(A\cap B)=1-\frac{1}{4}$

$\Rightarrow \frac{4-1}{4}$

$\Rightarrow \frac{3}{4}$

Now,

$P(A).P(B)=\frac{1}{2}\times \frac{7}{12}$

$=\frac{7}{24}\ne \frac{3}{4}$

$\Rightarrow P(A).P(B)\ne P(A\cap B)$

Therefore, $A$ and $B$ are not independent.

25. Given,

$P(A)=\frac{1}{4}$

$P(B)=\frac{1}{2}$

$P(A\cap B)=\frac{1}{8}$ Here,  $P$  (not $A$ not $B$ ) $=P(A^{\prime }\cap B^{\prime })=P(A\cup B{)}^{\prime }$

$\Rightarrow 1-P(A\cup B)$

$\Rightarrow 1-\left[P(A)+P(B)-P(A\cap B)\right]$

$\Rightarrow 1-\left[\frac{1}{4}+\frac{1}{2}-\frac{1}{8}\right]$

$\Rightarrow 1-\left[\frac{2+4-1}{8}\right]$

$\Rightarrow 1-\frac{5}{8}=\frac{8-5}{8}$

$\Rightarrow \frac{3}{8}$

24. Given,

$P(A)=0.3$

$P(B)=0.4$

$A$ and $B$ are independent

$\mathrm{(i)\; P}(A\cap B)=P(A).P(B)$

$\begin{array}{l}=0.3\times 0.4\\ =0.12\end{array}$

$\mathrm{(ii)\; P}(A\cup B)=P(A)+P(B)-P(A\cap B)$

$\begin{array}{l}P(A\cup B)=0.3+0.4-0.12\\ =0.58\end{array}$

$\mathrm{(iii)\; P}(A/B)=\frac{P(A\cap B)}{P(B)}=\frac{0.12}{0.4}=0.3$

$\mathrm{(iv)\; P}(B/A)=\frac{P(B\cap A)}{P(A)}=\frac{0.12}{0.3}=0.4$

23. Given,

$P(A)=\frac{1}{2}$

$P(A\cup B)=\frac{3}{5}$

$P(B)=p$

(i) We know that,

when $A$ and $B$ are mutually exclusive

$(A\cap B)=\varnothing$ $,P(A\cap B)=0$

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

$\Rightarrow \frac{3}{5}=\frac{1}{2}+p$

$\Rightarrow p=\frac{3}{5}-\frac{1}{2}=\frac{6-5}{10}=\frac{1}{10}$

(ii) when $A$ and $B$ are independent

$\begin{array}{l}P(A\cap B)=P(A).P(B)\\ P(A\cap B)=\frac{1}{2}\times p\end{array}$

Now,

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

$\Rightarrow \frac{3}{5}=\frac{1}{2}+p-\frac{1}{2}p$

$\Rightarrow \frac{3}{5}-\frac{1}{2}=\frac{2p-p}{2}$

$\Rightarrow \frac{6-5}{10}=\frac{p}{2}$

$\Rightarrow p=\frac{1\times 2}{10}=\frac{2}{10}=\frac{1}{5}$

$$22 . $P(F)=\frac{3}{10}$

$P(E\cap F)=\frac{1}{5}$

$P(E).P(F)=\frac{3}{5}\times \frac{3}{10}$

$=\frac{9}{50}\ne P(E\cap F)$

Here,  $P(E).P(F)\ne P(E\cap F)$ . $A$ and $B$ are not independent.

21. When die is thrown, the sample space

$S=\{1,2,3,4,5,6\}$

Let,  $A$ =the number is even $=\{2,4,6\}$

$B$ =the number is red $=\{1,2,3\}$

$\begin{array}{l}P(A)=\frac{3}{6}=\frac{1}{2}\\ P(B)=\frac{3}{6}=\frac{1}{2}\\ A\cap B=\left\{2\right\}\\ P(A\cap B)=\frac{1}{6}\\ \therefore P(A).P(B)=\frac{1}{2}\times \frac{1}{2}=\frac{1}{4}\ne P(A\cap B)\end{array}$

Hence,  $A$ and $B$ are not independent.

20. The sample space of given condition are

$S=\left\{\begin{array}{l}(H,1),(H,2),(H,3),(H,4),(H,5),(H,6)\\ (T,1),(T,2),(T,3),(T,4),(T,5),(T,6)\end{array}\right\}$

Let $A$ = head appear on the coin

$B$ = $3$ on the die

$A=\{(H,1),(H,2),(H,3),(H,4),(H,5),(H,6)\}$

$P(A)=\frac{6}{12}=\frac{1}{2}$

$B=\{(H,3),(T,3)\}$

$P(B)=\frac{2}{12}=\frac{1}{6}$

$A\cap B=\{(H,3)\}$

$i.e.,P(A\cap B)=\frac{1}{12}$

$P(A).P(B)=\frac{1}{2}\times \frac{1}{6}=\frac{1}{12}=P(A\cap B)$

Therefore, $A$ and $B$ are independent.