Walking 7/6 of his usual rate, a boy reaches his school 6 min early. Find his usual time to reach the school?  Choose the correct choice from the following choices:
A) 22 B) 28 C) 99 D) 42 E) 66

Rationale: "speed ratio = 1 : 7 / 6 = 6 : 7 time ratio = 7 : 6 1 - - - - - - - - 7 6 - - - - - - - - - ? = 42 m answer : d"
Answer: D