问题如下：
If an investor invest $500 in a fund which is supposed to provide a rate of return of 8% per year. If the investor want to earn $1,000, how many years should he invest?
选项：
A.7 years.
B.8 years.
C.9 years.
解释：
C is correct.
PV = -500; I/Y = 8; FV = 1,000; PMT = 0; CPT N = 9.01.
为什么payment=0 ？？
