A$104.000 selling price with $24000 down at 8 1/2% for 25 years results in a monthly payment of

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(See attached formula)
Loan Principal = 104,000 - 24,000 = 80,000
rate = 8.5 / 1,200  = 0.0070833333
time = 25 years = 300 months

Monthly Payment = 0.0070833333 + [ 0.0070833333 / ((1.0070833333)^300 -1)) ] *principal

Monthly Payment = 0.0070833333 + [ 0.0070833333 / ( 8.3104129461 -1)] * 80,000
Monthly Payment = 0.0070833333 + [ 0.0070833333 / ( 7.3104129461)] * 80,000
Monthly Payment = (0.0070833333 + 0.00096893750767) * 80,000
Monthly Payment = 0.00805227080767 * 80,000
Monthly Payment = 644.18

Source:
http://www.1728.org/calcloan.htm