Question
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 :
$$\eqalign{
& = \left( {\frac{{45}}{{20}}} \right){\text{ hrs}} \cr
& = \frac{9}{4}{\text{ hrs}} \cr
& = 2\frac{1}{4}{\text{ hrs}} \cr
& = 2{\text{ hrs }}15\min \cr} $$
Remaining time = (3 hrs - 2 hrs 15 min) = 45 min
Hence, required speed :
$$\eqalign{
& = \left( {\frac{{45}}{{45}}} \right){\text{ km/min}} \cr
& = 1{\text{ km/min}} \cr} $$
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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 :
$$\eqalign{
& = \left( {\frac{{45}}{{45}}} \right){\text{ km/min}} \cr
& = 1{\text{ km/min}} \cr} $$
Was this answer helpful ?
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