Suppose there is "3500" kj of energy available in the leaves. the rat eats the leaves, and the fox eats the rat. how many kilojoules of energy are available for the fox to use?

The fox will have roughly 35 kj of energy to use.

Explanation:

It must be noted that energy transfer at each trophic level is roughly 10 per-cent only, rest of the energy from the organism would get lost in metabolic activities.

So leaves with 3500 kj of energy would pass only 350 kj of energy to the rat which will roughly pass 35kj of energy to the fox.