Cesar purchased a room air conditioner for $450 and made a a down payment of $90 and financed the rest over 20 months. If the monthly payment was $22.80, what was the interest?

Answer 1

22.8 * 20 = 456

456 - 360 = ?

Answer 2

So loan = 450-90 = 360

let the monthly rate be r
22.8( 1 - (1+r)^-20)/r = 360

1 - (1+r)^-20 = 360r/22.8

very difficult equation to solve, I will use Wolfram
http://www.wolframalpha.com/input/?i=solve+1+-+%281%2Br%29%5E-20+%3D+360r%2F22.8

using the logical answer of r = .0236523..
the annual rate compounded monthly would be
.0236523(12) = .2838
or
appr 28.38 % per annum compounded monthly , very high!

check:
22.8(1+.0236523^-20)/.0236523
= 359.99994
= $360

Answer 3

You probably want to use Ms Sue's answer.
I did not read the question carefully and read it as finding the interest rate.