A model ecosystem has 10000 kJ in each square metre in a year at the first trophic level…

Step 1. (a) $5\%$: $10000\ \text{kJ}$, $500\ \text{kJ}$, $25\ \text{kJ}$, $1.25\ \text{kJ}$. The fourth level has less than $5\ \text{kJ}$. So there are 3 levels.

Step 2. (b) $10\%$: $10000\ \text{kJ}$, $1000\ \text{kJ}$, $100\ \text{kJ}$, $10\ \text{kJ}$, $1\ \text{kJ}$. The fifth has less than $5\ \text{kJ}$. So there are 4 levels.

Step 3. (c) $20\%$: $10000\ \text{kJ}$, $2000\ \text{kJ}$, $400\ \text{kJ}$, $80\ \text{kJ}$, $16\ \text{kJ}$, $3.2\ \text{kJ}$. The sixth has less than $5\ \text{kJ}$. So there are 5 levels.