Step-by-step explanation:
since Poisson distribution parameter is not given so we have to estimate it from the sample data. The average number of of arrivals per minute at an ATM is

So probabaility for
is

So expected frequency for
is
(or
.
Answer:
n=3
Step-by-step explanation:
I mean... in order to try each one you need to try it atleast once so.. 1 might be your answer