You need to reduce all costs to the sum that invested today will pay the cost when it falls due (i=10% corresponds here to i=0.1).

A cost of $180000 in year 8 has a present cost equivalent of

The present cost of the initial cost is itself

The net revenue (revenue less operating costs) over 15 years present value is (assuming this is accrued at the end of each year, if you want this paid continuously look through your notes for the appropriate formula):

(This last you will probably find in your notes as a present value of a future revenue stream formula)

The present value of the salvage value is:

.

When you combine all of the above make sure you use the correct signs for what are revenue and costs terms (costs are red, revenue black).

There are probably some mistakes in the above, but the basic idea is that you reduce everything to a sum which if invested today at the given interest rate will yield an equivalent payment/revenue stream.

CB