If the loan is paid in full at the end of that period, then amount to be paid back is $ 52085.13
Solution:
Given, An amount of $21,000 is borrowed for 10 years at 4% interest, compounded annually.
The loan is paid in full at the end of that period, how much must be paid back
Now, let us find compound interest first.

Here amount = 21000
r = rate of interest = 4%
t = time period = 10 years
By plugging in values we get,

So, compound interest on $21000 is $31085.13 approximately.
Now, amount to be paid back = loan amount + compound interest
Amount to be paid back = 21000 + 31085.13 = 52085.13
Hence, amount to be paid back is $ 52085.13