The density of water is 62.4 lb/ft3. if a swimming pool holds 10,000 gallons of water, the weight of water in the pool is lbs. (231 in3 per gallon; 1728 in3 per ft3) (only place the numerical value in blank) (round to the nearest whole number)

In this case, you are given the water density (62.4lb/ft3) and the water volume (10,000 gallons). But the unit used in the density and gallon is different, so you need to convert it. The calculation would be:

mass= density * volume
mass= 62.4 lb/ft3 * 10,000 gallons * (231 inch3/gallon) * (1 ft3/ 1728 inch3)
mass= 83,416.66 lbs= 81417 lbs