The payback period is always calculated using cash flows, and therefore the net cash flow each year is $2,500.
The accounting rate of return is always calculated using profits, and profits are always after subtracting depreciation from the cash flows.
I do suggest that you watch my free lectures on this (and, if necessary, the relevant lectures from Paper MA (was F2) because this is revision from Paper MA).
The lectures are a complete free course for Paper FM and cover everything needed to be able to pass the exam well.