A person wishes to reach his destination 90 km away 3 hours but for the first half of the journey his speed was 20 km/hr. His average speed for the rest of the journey should be :

Solution (By Examveda Team)

Time taken to travel 45 km :
$$\eqalign{ & = \left( {\frac{{45}}{{20}}} \right){\text{ hrs}} \cr & = \frac{9}{4}{\text{ hrs}} \cr & = 2\frac{1}{4}{\text{ hrs}} \cr & = 2{\text{ hrs }}15\min \cr} $$
Remaining time = (3 hrs - 2 hrs 15 min) = 45 min
Hence, required speed :
$$\eqalign{ & = \left( {\frac{{45}}{{45}}} \right){\text{ km/min}} \cr & = 1{\text{ km/min}} \cr} $$