To start you would want to find how much wax it takes to make one candle. So 5x4x15 will equal the cm^3 needed to make each candle. That's 300cm^3 of wax for each candle. If they used 5110cm^3, then just divide 5110 by 300. That will give you 17.03, but I'm assuming they want you to to solve to a whole number, so they could get 17 candles out of 5100 cm^3 of wax, and have 10cm^3 remaining.
