The number of calls coming into a PBX has a mean of 120 call

The number of calls coming into a PBX has a mean of 120 calls per hour. What is the probability of no calls in a one-minute interval?

Solution

Possion Distribution
PMF of P.D is = f ( k ) = e- x / x!
Where   
= parameter of the distribution.
x = is the number of independent trials

PBX has a mean of 120 calls per hour
Per minute the mean rate will be 120/60 = 2 calls

P( X = 0 ) = e ^-2 * 2^0 / 0! = 0.1353