We will have the following:
We will have that the value at the end will be $180 000.
The number of periods will be 15*12 = 180.
The interest per month is 0.05/12 = 0.0042
Now:
Here "A" is the value at the end, "P" will be the monthly payments, "i" the monthly interest rate and "n" the number of periods. So:
So, they will need to deposit approximately $671.2 each month.