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

Since the boys now walks at $\frac{5}{7}th$of usual speed

So, we take $\frac{7}{5}th$of his usual time 

Extra time $\Rightarrow (\frac{7}{5}-1)\times usualtime=\min$

$\Rightarrow \frac{2}{5}\times usualtime=6$

Usual Time = 15 mins