A stone dropped from a tower takes 5 s to reach the ground…

Step 1. Height. $\tfrac12 \times 10\ \text{m/s}^2 \times (5\ \text{s})^2 = 125\ \text{m}$.

Step 2. After 4 s. $\tfrac12 \times 10\ \text{m/s}^2 \times (4\ \text{s})^2 = 80\ \text{m}$.

Step 3. Last second. $125\ \text{m} - 80\ \text{m} = 45\ \text{m}$.