You borrowed $25 from your friend. You paid him back in full after 6 months. He charged $2 for interest. What was the annual simple interest rate that he charged you?

Annual interest

6 months = 1/2 year

so 2 dollars=1/2 of yearly interest

4 dollars is yearly

4 out of 25

4/25 = x%

percent means parts out of 100

x%=x/100

4/25=x/100

multiply both sides 100

16=x

answer is 16%