Question

How much interest will she earn in 3 years?

Answer 1

For this, the equation is 
90× 1.13³=y
X was replaced with y. Since she is gaining, you have to remember she is going to have 100 percent of what he had before, she will not lose any.
90× 1.13= 129.86
Now, we need to find the amount of interest she gained, so we have to subtract 90 from the total.
129.86-90=39.86
We can also find the answer by completing a small chart.
The first column will be the year. The second will be the amount. The third will be the interest for the year.
1                                                90                                                11.7
2                                                101.7                                           13.22
3                                                 114.92                                        14.94
The total would round to 39.86, so her interest for the 3 year would be 39.86, with her having 129.86 altogether.

Answer 2

At the end of each year, the balance is (1 +0.13) times the amount at the beginning of the year. After 3 years, the balance will be
.. $90*1.13^3 = $129.86

Of that balance, $90 is the original deposit, so the interest earned is
.. $129.86 -90.00 = $39.86