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 :

Answer: Option C

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