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

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%