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:

Explanation:-

When the athlete walks at ⁵/₄ of his usual speed then he takes ⁴/₅ of his usual time (As Speed ∝ (¹/_Time)). and he saves 5 minutes.
→ Usual time - (⁴/₅)*usual time = 5 minutes;
→( ¹/₅ )* Usual time = 5 minutes;
Usual time = 25 minutes.