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]假設今年收入為i元，而今年存下s元，
亦即今年花費為i-s元，明年可花費s(r+1)元。題幹要求s/i的值。
依據題意，明年花費是今年花費的一半，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)，
選E。