Current estimates suggest that 75 of the homebased computers

Current estimates suggest that 75% of the home-based computers in a foreign country have access to on-line services. Suppose 20 people with home-based computers were randomly and independently sampled. Find the probability that 9 of those sampled currently do not have access to on-line services..

Solution

Binomial Distribution

PMF of B.D is = f ( k ) = ( n k ) p^k * ( 1- p) ^ n-k
Where   
k = number of successes in trials
n = is the number of independent trials
p = probability of success on each trial

P( X = 9 ) = ( 20 9 ) * ( 0.25^9) * ( 1 - 0.25 )^11
= 0.0271