The density of a substance is defined as the mass of the substance divided by the volume of the substance that is being considered.
Density = mass/volume
The density of petrol is 800 kg/m^3. One m^3 of petrol weighs 800 kg. We have to determine the volume of 3200 kg of petrol.
Using ratios we have 800 kg : 1 ^3, multiplying both sides by 4 gives 3200 kg : 4 m^3. The volume of 3200 kg of petrol is 4 m^3.
The volume of 1600 kg of petrol is 1600/800 = 2 m^3.
As the density of lead is 11400 kg/m^3, the mass of 2 m^3 is 11400*2 = 22800 kg. The mass of lead with the same volume as 1600 kg of petrol is 22800 kg.