Walking 7/6 of his usual rate, a boy reaches his school 3 min early. Find his usual time to reach the school?  Choose the correct choice from the following options:	A) 21	B) 28	C) 99	D) 77	E) 66

Rationale: "speed ratio = 1 : 7 / 6 = 6 : 7 time ratio = 7 : 6 1 - - - - - - - - 7 3 - - - - - - - - - ? = 21 m answer : a"
Answer: A