Bunuel wrote:

Mike makes 7 phone calls on the first day of a local fundraising drive and increases the number of calls he makes by 4 calls each day for the 10-day drive. What is the average number of calls he makes during the drive?

A. 25

B. 27

C. 28

D. 50

E. 54

After the first day of 7 calls, he increases each day's number of calls by 4 for 9 days. It's an arithmetic progression, whose average =

\(\frac{FirstTerm+LastTerm}{2}\)

First term = 7

Last term* = (7 + (9*4)) = (7 + 36) = 43

Or just list terms: 7,11,15,19,23,27,31,35,39,43

\(\frac{7+43}{2}=25\)

Answer A

\(A_{n} = A_1 + (n-1)d\)

\(A_{10} = 7 + (9)*(4) = 43\)
_________________

In the depths of winter, I finally learned

that within me there lay an invincible summer.

-- Albert Camus, "Return to Tipasa"