Question

It is known that 60% of mice inoculated with a serum are protected from a certain disease. If 5 mice are inoculated, find the probability that none contract the disease .

Answer 1

Let X be the number of mice that contract the disease .
Then, X follows a binomial distribution with n =5.
Let p be the probability of mice that contract the disease.

\[\therefore p = 0 . 4 \text{ and }  q = 0 . 6\]
\[\text{ Hence, the distribution is given by} \]
\[P(X = r) = ^{5}{}{C}_r \left( 0 . 4 \right)^r \left( 0 . 6 \right)^{5 - r} , r = 0, 1, 2, 3, 4, 5\]
\[ P(X = 0) = ^{5}{}{C}_0 \left( 0 . 4 \right)^0 \left( 0 . 6 \right)^{5 - 0} \]
\[ = (0 . 6 )^5 \]
\[ = 0 . 0778\]