With mortgages, we want to find the monthly payment required to totally pay down a borrowed principal over the course a number of payments.The standard mortgage formula is:M=P[i(1 +i)^n ] / [ (1 +i)^n- 1]

Where M is the monthly payment.i=rate/12 and n = no. of months.

$75,000 less 20% = $60,000 = P.

For our $60,000 mortgage at 12% compounded monthly for 30 years, we would first solve for i as

i= 0.12 / 12 = 0.01 andnas 12 x 30 = 360 monthly payments .

Next we would solve for (1 +i)^n = (1.01)^360, which yields 35.94964133.

Now our formula readsM=P[i(35.949641333)] / [ 35.94964133- 1] which simplifies to

M=P[.01 x 35.94964133] / 34.94964133 or

M= $60,000 x 0.010286126 = $617.17

You can check this answer using a mortgage calculator like this one: Mortgage Calculator