The rate of interest is 7.25 % per year
Solution:
Given that Johnson borrowed $8000 for 4 years to make home improvements
He repaid a total of $ 10320
To find: Rate of interest
The simple interest is given as:
![S.I = ( p * n * r)/(100)](https://img.qammunity.org/2021/formulas/mathematics/middle-school/fyfk84c8lv4afm75gt27ou48dqyc3h7mac.png)
Where "p" is the principal and "n" is the number of years and "r" is the rate of interest
We know that,
Total amount = principal + simple interest
10320 = 8000 + simple interest
Simple Interest = 10320 - 8000
Simple interest = 2320
Substitute S.I = 2320 and n = 4 in simple interest formula
![2320 = (8000 * 4 * r)/(100)\\\\2320 = 80 * 4 * r\\\\2320 = 320r\\\\r = (2320)/(320)\\\\r = 7.25](https://img.qammunity.org/2021/formulas/mathematics/middle-school/95qpx01t2zgs0kg5qc0h5n9d3179jty1vk.png)
Thus rate of interest is 7.25 % per year