Walking 4/3 of his usual rate, a boy reaches his school 4 min early. Find his usual time to reach the school?  Choose the correct choice from the following:
A) 22  B) 16  C) 27  D) 28  E) 20

Rationale: "speed ratio = 1 : 4 / 3 = 3 : 4 time ratio = 4 : 3 1 - - - - - - - - 4 4 - - - - - - - - - ? 16 m . answer : b"
Answer: B