Running at 5/4 of his usual speed, an athlete improves his timing by 5 minutes. The time he usually takes to run the same distance is:

Answer: Option C

When the athlete walks at $$\frac{5}{4}$$ of his usual speed then he takes $$\frac{4}{5}$$ of his usual time $$\left(\text{As}\text{Speed}\propto \frac{1}{\text{Time}}\right)$$    and he saves 5 minutes.
→ Usual time - $$\frac{4}{5}$$ × usual time = 5 minutes
→ $$\frac{1}{5}$$ × Usual time = 5 minutes
Usual time = 25 minutes.