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)
[解]假设今年收入为i元，而今年存下s元，
亦即今年花费为i-s元，明年可花费s(r+1)元。题干要求s/i的值。
依据题意，明年花费是今年花费的一半，可以列式：
s(r+1)=(1/2)(i-s)，得到s=[1/(2r+3)]i，所以s/i=1/(2r+3)，
选E。