a) If 100 joules of energy are available at the third trophic level, then the producer level must have 10,000 joules of energy, as only 10% of energy is transferred at each step. Energy decreases through trophic levels due to losses in metabolic activities like respiration, movement, and heat.

b) No, it is not possible to add two more trophic levels before the fourth level, because the energy available would become too small to sustain additional organisms efficiently in the food chain.