If we apply the reverse process of increasing (as in compound percentage) then we should have what we need.
Suppose the initial sum borrowed was P (principle sum)
Suppose the related interest was
Then 1 years interest is
Add this to the initial deposit and you have:
Factor out the P and you
Do this over
All we need to do is change
Using the above and substituting the values in the question we have:
divide both sides by $20000
Note that you can treat units of measurement the same way you do numbers. Very useful in applied maths or physics.
Take logs of both sides
In the 'olden days'