不太明白PREP上的一道数学题

Alice's take-home pay last year was the same each month, and she saved the same fraction of her take-home pay each month.  The total amount of money that she had saved at the end of the year was 3 times the amount of that portion of her monthly take-home pay that she did not save.  If all the money that she saved last year was from her take-home pay, what fraction of her take-home pay did she save each month?

(A) 1/2

(B) 1/3

(C) 1/4

(D) 1/5

(E) 1/6

麻烦大家帮忙解答一下~

设每月收入为M 存钱比例为S

则有12*M*S=3*M*（1-S）

整理一下 就可以算出S=1/5

哈哈，明白了，太感谢了~