On february 1, 1977, john deposited $2250 into a savings account paying 5.76% interest, compounded quarterly. if he hasn't made any additional deposits or withdrawals since then, and if the interest rate has stayed the same, in what year did his balance hit $4500, according to the rule of 72?

John's effective annual rate is about
  (1 +.0576/4)^4 -1 ≈ 5.8856%
According to the "rule of 72", John's money will have doubled in
  72/5.8856 = 12.23 years

John's balance will be $4500 in 1989.


Since you're only concerned with the year (not the month), you don't actually need to determine the effective annual rate. The given rate of 5.76% will tell you 72/5.76 = 12.5 years. The actual doubling time is closer to 12.12 years, so using the effective rate gives results that are closer, but "good enough" is good enough in this case.

31 is prime. and it’s prime factorization is just 31.