Walking 5/7 of his usual rate, a boy reaches his school 6 min late. Find his usual time to reach the school?

Since, the boys now walks at 5/7 of usual speed, so he will take 7/5 of his usual time.
⇒ Extra time = (7/5 - 1) x Usual time = 6 min (known)
⇒ 2/5 x Usual time = 6
∴ Usual time = 15 min