求助一道gwd5

stevegt

Q28:

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)

I have no idea....please help me, thanks.

meimei5656

设今年收入为R,储蓄比率是x，则今年的支出为R(1-x)，储蓄为Rx,明年的储蓄总额为R*x*(1+r), 则有等式R*x*(1+r)=0.5*R*(1-x),化简得x=1/(3+2r)

wanders

同意  关键是设定储蓄比率fraction of his income.

luoye1

but for each dollar that he saves this year, he will have 1 + r dollars available to spend.

我竟然把这句话理解成第二年，他又1+r这么多钱可以花。。。。