Travelling at 3/5^{\text{th}} of his usual speed, a man is late by 20 minutes. What is the usual time if he travels with his usual speed?

Explanation

Speed and time are inversely proportional. Since speed becomes \frac{3}{5} of usual, time becomes \frac{5}{3} of usual. The extra time is \frac{5}{3}T - T = \frac{2}{3}T = 20 minutes. Thus, the usual time T = 30 minutes.

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