Two boys A and B start at the same time to ride from Delhi to Meerut, 60 km away. A travels 4 km an hours slower than B, B reaches Meerut and at once turns back meeting A, 12 km from Meerut. A's rate was :

Let A's speed = x km/hr
Then, B's speed = (x + 4) km/hr
Clearly, time taken by B to cover (60 + 12) i.e., 72 km
= Time taken by A to cover (60 - 12) i.e., 48 km
$$\eqalign{ & \therefore \frac{{72}}{{x + 4}} = \frac{{48}}{x} \cr & \Rightarrow 72x = 48x + 192 \cr & \Rightarrow 24x = 192 \cr & \Rightarrow x = 8 \cr} $$
Hence, A's speed = 8 km/hr