[問題]GMAT Prep (Test1)

[QQA]

If each term in the sum a1+a2+......+an is either 7 or 77 and the sum equals 350, which of the following could be euqal to n?

(1) 38
(2) 39
(3) 40
(4) 41
(5) 42

這題要怎麼算出 n =40 ??