Money are really best dealt with as integers, any time you'd use a non-integer number, use some fixed multiple that makes it an integer, then divide by the excess factor at the end of the calculation. For instance computing 2.15% yearly interest on a bank account might be done as follows:
DaysInYear = 366
InterestRate = 215
DayBalanceSum = 0
for each Day in Year
DayBalanceSum += Day.Balance
InterestRaw = DayBalanceSum * InterestRate
InterestRaw += DaysInYear * 5000
Interest = InterestRaw / (DaysInYear * 10000)
Balance += Interest
Balance should always be expressed in the smallest fraction of currency that we conventionally round to, like 1 yen or 1/100 dollar. Adding in half of the divisor before dividing effectively turns floor division into correctly rounded division.
