GWD 5-28

AlphaBeta

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

bluespecter1

设收入是I，save的比例是F，则明年会有I * F * (1+r)，这个值是 SPENDS的1/2，即 1/2 * I*(1-F) 。 所以I * F * (1+r) = 1/2 * I*(1-F) => F = 1/(2r+3). 选E。

婉如

我觉得不需要设什么比例，直接设储蓄是S,收入是R，则由原题得(R-S)/2=S(r+1),得出S/R= 1/(2r+3)

liaotao85

QUOTE:

(1+r)*aX=(1-a)X*0.5    a=1/(3+2r)

他今年存的钱，下一年的有利息，是存的1＋r倍，这个是他下一年可以花的钱，应该等于

他今年花的钱的一半。

请问利息是从哪里看出来有的？？

文中没说阿