Walking 7/6 of his usual rate, a boy reaches his school 4 min early. Find his usual time to reach the school?  Answers
A) 11 B) 15 C) 16 D) 28 E) 19

Rationale: "speed ratio = 1 : 7 / 6 = 6 : 7 time ratio = 7 : 6 1 - - - - - - - - 7 4 - - - - - - - - - ? è 28 m answer : d"
Answer: D