Question

At what rate must $287.50 be compounded annually for it to grow to $572.86 in 8 years?

Answer 1

Using the compound interest formula,
572.86=287,50(1+i)^8
(1+i)^8=572.86/287.50=1.992557
1+i=(1.992557)^(1/8)=1.0899996
=>
i=1.0899996-1=9%

Note: This problem can also be solved in the head using the rule of 72.
Since after 8 years, the amount is roughly doubled, we can say that the interest rate is approximately 72/8=9%