OK the question is "The Samuels have a mortgage of $123000 amortized over 25 years at 7.25% compunded monthly. After the original 4 year term, the mortgage is renewed at 6.5% compunded monthly. Calculate the new monthly payment."

The answer is $836.90, But I can't get that answer. This is what I did:

[123000*(0.0725/12)] / [1-(1+(.0725/12))^-300)

=$889.05 (thats the monthly payment for the 4 years, I need to extract the Principal now)

P= 889.05- (123000*(0.0725/12)

=889.05-743.13

=$145.92 (this is the ammount of principal payed each month, multiply that for 4 years monthly (12*4=48 ))

$145.92*48= $7004.16 (now subtract from 123000)

123000-7004.16 = 115995.84

Now that that is done...THE NEW PAYMENT! (which i can't get correct )

115995.84*(0.065/12) / [1-(1+(0.065/12))^-252

and that = $844.87 but the answer is $836.90

Does anyone know this math and can help me? I don't know where I went wrong. I followed the steps :S...