Walking 3/2 of his usual rate, a boy reaches his school 4 min early. Find his usual time to reach the school?  Choose the correct option.
A) 12  B) 99  C) 27  D) 28  E) 20

Rationale: "speed ratio = 1 : 3 / 2 = 2 : 3 time ratio = 3 : 2 1 - - - - - - - - 3 4 - - - - - - - - - ? 12 m . answer : a"
Answer: A