问题如下：
Isabel Corp. invested a machinery with initial capital of $7,000 at the end of last year. It will generate cash inflows of $2,000 for the following four years. At the end of fourth year, the machinery is expected to have a residual value of $900. Assume the required rate of is 8%, the profitable index (PI) of this machinery is closest to:
选项：
A. 1.40
B. 1.24
C. 1.04
解释：
C is correct. PI equals Present value of future cash flows dividing initial investment. It can also be expressed by PI=1+NPV/CF0. NPV can be computed by a calculator with inputs: CF0=-7,000, CF01=2,000, F01=3, CF02=2,900, I=8. Compute the NPV=285. Therefore PI=1+285/7,000=1.04.
这里剩余价值是什么意思？为什么要加上去？