Western National Bank is considering opening a drive-thru window for customer service. Management estimates that customers will arrive at the rate of 15 per hour. The teller who will staff the window can service customers at the rate of one every three minutes. The average number in the waiting line is

9 minutes

Explanation:

The average waiting time in line, w, is given by the formula

w = a / u(u - a)

Where,

w = average waiting time in line

a = rate of customers per hour

u = rate at which customers are being serviced.

The rate at which customers are being serviced is 1 in every 3 minutes, and if we convert minute to hour, we end up with 20 in every hour. So, on solving

w = 15 / 20(20 - 15)

w = 15 / 20(5)

w = 15 / 100

w = 0.15 hour.

On converting this back to minutes, we multiply by 60 and have

0.15 * 60 = 9 minutes

Therefore, the waiting time is 9 minutes

hello the answer is d