# Thread: Interest paid in a payment

1. ## Interest paid in a payment

Hello, I had a tough question recently in class that I couldn't figure out how to do on my financial calculator (HP10BII)

Here's the problem: You are buying a house for \$150,00 with a 10% down payment and a fixed-rate mortgage for the remainder at 8% for 20 years with MONTHLY payments. How much interest is paid on the 20th payment?

Here's what I do and where I get stuck...

150,000 (.1) = 15,000
150,00 - 15,000 = 135,000 which is my PV (Present Value)
now I have my interest at 8%/12 = 0.667 (Interest)
and for my monthly payments it is 20x12 = 240 (# of payments)

Now I know there some function to figure out how much interest is paid on the 20th payment but I don't know what I enter into the calculator to get it. So for a run-down here it what I entered into my calculator

-135,000 [PV]
0.667 [I]
240 [N]
and now I'm stuck

Any help is appreciated!

2. Ponder this from the back.

Assume a payment of 1.

Principle in the last payment is v.
Principle in the 2nd to last payment is v^2.
Principle in the 3nd to last payment is v^3.

etc.

Interest in the last payment is 1-v.
Interest in the 2nd to last payment is 1-v^2.
Interest in the 3nd to last payment is 1-v^3.

etc.