Walking 6/5 of his usual rate, a boy reaches his school 4 min early. Find his usual time to reach the school? Select the correct option:
A) 22 B) 99 C) 27 D) 24 E) 20

Rationale: "speed ratio = 1 : 6 / 5 = 5 : 6 time ratio = 6 : 5 1 - - - - - - - - 6 4 - - - - - - - - - ? 24 m . answer : d"
Answer: D