During a busy hour, the number of calls per hour for each of the 12 cells of a cellular cluster is 2220, 1900, 4000, 1100, 1000, 1200, 1800, 2100, 2000, 1580, 1800 and 900. Assume that 75% of the car phones in this cluster are used during this period and that one call is made per phone. Find the number of customers in the system.
a. 15200
b. 16200
c. 17200
d. 18200
Answer: b