Heating a swimming pool

If electricity costs 3.9 cents per MJ (1 MJ = 1 megajoule = 106 J), how much does it cost to heat a 26000-gallon swimming pool from 10 °C to 18 °C?

Converting gallons to cm³ gives

Water has a density of 1 gram per cubic centimeter, so the mass of the water is 9.8 × 107 g. One joule is sufficient to heat 0.24 g by 1 °C, so the energy needed to heat the swimming pool is

The cost of the electricity is (3.3 × 10³ MJ)($0.039/MJ)=$130.