Answer:
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
