【問】PP-PS1-62

請教大家, 

PP-PS1-62

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

Ans: D

寫題時我選 C, 看了很久,就是找不到題意能發展到1/5的比例,還請諸位幫忙

感謝萬分

设每月薪水为X, 每月存款占薪水的比例是R (0<R<=1)

12*R*X=3*X*(1-R) --> R=1/5