Question

Joe's electric made 18 customer calls last week and 25 calls this week. How many calls must be made next week in order to maintain an average of at least 21 calls for the three week period at least __ calls must be made to maintain an average of 21.
Is the answer 63 calls?
Ms. Sue
No. He must make 63 calls total in three weeks.

21 * 3 = 63

18 + 25 = 43 -- He's already made 43 calls in two weeks.

63 - 43 = 20