Four years ago you invested some money at 10% interest. You now have $839.23 in the account. If the interest was compounded yearly, how much did you invest 4 years ago?
PV = FV / [1 + R]^N
PV = 839.23 / 1.10^4
PV =839.23 / 1.4641
PV = 573.21 - what you invested 4 years ago.