Use your cash flow key/entry point. CF0 = -$1000, Cf1 through Cf4 = 0, Cf5 through Cf8 = $80 or set Cf5 to 80 and use the repitition key set at 4 at Cf5. Go to NPV and enter RR of 28%, then solve for NPV.
For the interest payments, do (80*1.06^7) + (80*1.06^6) + (80*1.06^5) + (80*1.06^4)
Add these two? Little too much drinking and haven't been in school for some time, hopefully that works. Do an acid test on this value, it should be north of 1000.