A walk at a uniform rate of 4 km an hour; and 4 hr after his start, B bicycles after him at the uniform rate of 10 km an hour. How far from the starting point will B catch A ?

Answer: Option D

Distance covered by A in hrs = (4 × 4) km = 16 km
Relative speed of B w.r.t. A = (10 - 4) km/hr = 6 km/hr
Time taken to cover 16 km at relative speed :
$$\begin{array}{rl}& =\left(\frac{16}{6}\right)\text{ hrs}\\ & =\frac{8}{3}\text{ hrs}\end{array}$$
Distance covered by in $$\frac{8}{3}$$ hrs
$$\begin{array}{rl}& =\left(10\times \frac{8}{3}\right)\text{ km}\\ & \text{ = }\left(\frac{80}{3}\right)\text{ km}\\ & \text{ = 26}\text{.7 km}\end{array}$$