Question

A bag contains 7 red, 5 white and 8 black balls. If four balls are drawn one by one with replacement, what is the probability that all are white ? 

Answer 1

Let X be the number of white balls drawn when 4 balls are drawn with replacement. 
  X follows binomial distribution with n = 4.

\[p = \text { Probability for a white ball } = \frac{\text{ No of white balls }} {\text{ Total no . of balls}} \]
\[ = \frac{5}{20}\]
\[ = \frac{1}{4}\]
\[\text{ and } q = 1 - p = \frac{3}{4}\]
\[P(X = r) =^{4}{}{C}_r \left( \frac{1}{4} \right)^r \left( \frac{3}{4} \right)^{4 - r} \] 

\[\text{ Prob that all are white } = P(X = 4) = ^{4}{}{C}_4 \left( \frac{1}{4} \right)^4 \left( \frac{3}{4} \right)^{4 - 4} = \frac{1}{256}\]