Train A is 150 m long at 54 km/h. Train B is 250 m long at 36 km/h…

Step 1. The distance. $150\ \text{m} + 250\ \text{m} = 400\ \text{m}$.

Step 2. (a) Closing at $15\ \text{m/s} + 10\ \text{m/s} = 25\ \text{m/s}$: $t = \dfrac{400\ \text{m}}{25\ \text{m/s}} = 16\ \text{s}$.

Step 3. (b) Closing at $15\ \text{m/s} - 10\ \text{m/s} = 5\ \text{m/s}$: $t = \dfrac{400\ \text{m}}{5\ \text{m/s}} = 80\ \text{s}$.