Explain why, a food chain usually cannot have more than three or four steps.

Name and state the law given by Lindeman which tells us how much energy entering a particular trophic level of organisms is available for transfer to the next higher trophic level. 

In the food chain given below, if the amount of energy available at the fourth trophic level is $5 kJ$, what was the energy available at the producer level?
Grass $\to$ Grasshopper $\to$ Frog $\to$ Snake $\to$ Hawk