We have a purchase of $1,300,000.
The downpayment is $150,000 and the rest is financed.
We can calculate the amount that is owed as:

This amount will be paid in equal amounts, monthly for 20 years.
The interest rate is 16% compounded quarterly.
We have to start by converting the interest rate in a monthly-compounded equivalent rate.
A rate of 16% compounded quarterly (m=3) will be equivalent to a monthly rate r. We can calculate r as:

We then can calculate the payments as an annuity with r = 0.013 and 20*12 = 240 payments.
We can calculate the amount he will pay each month as:

Answer: the monthly payment is approximately $15654.45.