Since the bucket has already a mass of 2,000 kg, therefore the maximum mass of gravel would only be:
15,000 kg – 2,000 kg = 13,000 kg gravel
Solving for volume:
volume = 13,000 kg / (1,500 kg / 1 m^3)
volume = 8.67 m^3
Therefore about 8.67 cubic meter of gravel can be safely loaded by the crane.
