Each cash inflow/outflow is discounted back to its present value (PV). Then they are summed. Therefore NPV is the sum of all terms , where
t - the time of the cash flow
i - the discount rate (the rate of return that could be earned on an investment in the financial markets with similar risk.)
- the net cash flow (the amount of cash, inflow minus outflow) at time t (for educational purposes, is commonly placed to the left of the sum to emphasize its role as (minus the) investment.