An olympic-size pool is 50.0 m long and 25.0 m wide. a. how many gallons of water (density = 1.0 g/ml) are needed to fill the pool to an average depth of 4.8 ft? b. what is the mass, in kg, of water in the pool?

1. To calculate the volume of the pool you need to convert the length, width and height into one same unit. The volume would be:
volume= length * width * height
volume= 50m *25m *(48ft *0.3048m)= 18288m^3
volume in gallons= 18288m^3 * (264.172 gallons/m^3)= 483.118 gallons

2. The mass would be found by multiplying density with volume. You will need to some unit conversion to get the mass in kilogram. Remember that 1g= 1000kg and 1m^3= 1,000,000ml
mass= volume * density
mass= 18288m^3 * (10^6 ml^3/m^3) * 1.0 g/mL / (1000g/kg)= 18,288,000 kg

Yes you are correct use the scientific method