有一道数学题算不出来了～

DorothyHong

今天做题有些脑残了…所以数学题做不出 希望大家帮忙指点下 谢谢拉！

This year Henry will save a certain amount of his income, and he will spend the rest. Next year Henry will have no income, but for each dollar that he saves this year, he will have 1+r dollars available to spend. In terms of R, what fraction of his income should Henry save this year so that next year the amount he was available to spend will be equal to half the amount that he spends this year?

A. 1/(r+2)
B. 1/(2r+2)
C. 1/(3r+2)
D. 1/(r+3)
E. 1/(2r+3)

设来设去列来列去就是没个结果～谁告诉我个方法？

谢谢谢谢～好傻阿感觉自己。

Kevin4U

设第一年应该save x% of 总收入，则可以推出
（1-x%）=(1+r)*x%
  x%=1/ （2+r）

leeyeuktung

'equal to half the amount that he spends this year'
所以等式应该是, 设第一年应该存款的比例为x, 那么 (1-x)*1/2=(1+r)*x
x=1/(2r+3) 选E

G_will

1L貌似不对吧。

收入为I，比例设为x，则有

xI(1+r)=(1-x)I/2

得x=1/(2r+3)

fly12404

同意是E

HelenW

同意E

michaelwong

E正解，OG里面的题

DorothyHong

阿阿 终于明白了 我是设的有问题 唉 脑子搭住了
谢谢哈

DorothyHong

我是在gwd里做到的 每次gwd数学都错很多 感觉很受打击～