Sound travels at 343 m/s in air, 1498 m/s in water and 5950 m/s in iron.

Step 1. (a) Air: $\dfrac{2500\ \text{m}}{343\ \text{m/s}} = 7.29\ \text{s}$. Water: $\dfrac{2500\ \text{m}}{1498\ \text{m/s}} = 1.67\ \text{s}$. Iron: $\dfrac{2500\ \text{m}}{5950\ \text{m/s}} = 0.42\ \text{s}$.

Step 2. (b) $\dfrac{5950\ \text{m/s}}{343\ \text{m/s}} = 17.3$.