Question

Walking at $\frac{7}{8}$th of his usual speed, a man reaches late at his office by $15$ min. Find his usual time (in minutes).

Answer 1

Speed becomes $\frac{7}{8}$ of usual speed, it means time become $\frac{8}{7}$ of usual time taken.

[ As speed is inversely proportional to time taken when distance is constant. ST = D ]
So, $\frac{8}{7}$ of usual time = usual time $+15$ min
or, $\frac{8}{7}$ of usual time - usual time $=15$ min
or, $\frac{1}{7}$ of usual time $=15$ min
So, usual time $=15\times 7$
or, Usual time $=105$min $=1hour45$ min.