When you say "no interest-free mortgages", this would mean that there is an interest. However, the problem does not state any interest rate. So, let's just assume a value for the interest rate. In US, the current 15-year mortgage rate is 2.88%. (source:https://ycharts.com/indicators/15_year_mortgage_rate)
Then,
F = P (1 + i)^n
where F is the future worth, P is the present worth, i is the interest and n is the number of years.
F = $1000 (1+0.0288)^15
F = $1,530.96
On the event that there is no interest (which is unlikely), the amount would still be $1000.
