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Net Present Value (NPV) of a time series of cash flows (incoming and outgoing), is defined as the sum of the present values of the individual cash flows.
\begin{align} NPV &=-C_{0}+\sum_{i=1}^{t}\frac {C_{i}}{(1+r)^{i}}\\ \\ PI &=\frac{\sum_{i=1}^{t}\frac {C_{i}}{(1+r)^{i}}}{C_{0}}\\ \end{align}