Elva invested a sum of money in an account paying compound interest at 4% per year.
After 3 years, there was $89 989.12 in her account. How much did Elva invest in the account? Answer $ ...

1. Write the compound interest multiplier for 3 years at 4% per year.

An increase of $4\%$ each year gives a multiplier of
$1+\frac{4}{100}=1.04.$
For 3 years, the total multiplier is
$1.04^3.$

2. Form the reverse percentage calculation for the original investment.

Let the original investment be $P$.

$P(1.04)^3=89989.12$

So
$P=\frac{89989.12}{1.04^3}.$

3. Calculate the original investment.

$P=\frac{89989.12}{1.04^3}$
$P=\frac{89989.12}{1.124864}$
$P=80000$