Well, do it!

You have i = 0 .12, then v = 1/1.12

This gives: 4000v + (4000 - 150)v^2 + (4000 - 150*2)v^3 + ... + (4000 - 150*14)v^15 as the original value of the loan.

A little algebra gives:

4000v + 4000v^2 + 4000v^3 + ... + 4000v^15 - (150*v^2 + 150*2*v^3 + ... + 150*14*v^15)

Just a hair more algebra helps quite a bit.

4000v(1 + v + v^2 + ... + v^14) - 150*v^2(1 + 2*v + ... + 14*v^13)

What say you? Can you add it up from there?