Assume that on an average one telephone number out of 15, called between 3 p.m. on weekdays, will be busy. What is the probability that if six randomly selected telephone numbers are called, at least 3 of them will be busy?
The probability that the called number is busy is 
Using Bernoulli’s Trial we have,
P(Success=x) = nCx.px.q(n-x)
x=0, 1, 2, ………n and q = (1-p), n =6
The probability that at least three of them will be busy is:-
P(0) + P(1) + P(2) + P(3)
⇒ 6C0(
)0(
)6 + 6C1(
)1(
)5 + 6C2(
)2(
)4 + 6C3(
)3(
)3
⇒ 1 – (
)4.(
)
1