Because: the load capacity of a car is 1/3 of that of a big car.

Let's assume that the load capacity of trucks and cars is 3X tons and x tons respectively.

Six big trucks and nine cars carry 54 tons of goods at a time.

Then there is the equation: 6*(3X)+9X =54.

27X=54

X=2

The load capacity of trucks and cars is 6 tons and 2 tons respectively.