Driving Speed

Level pending

Robert always goes to work and goes home in his car.

One day, he went to work with an average speed of 90 km/h. At night, there was a traffic congestion. His average speed then was 45 km/h.

What was his average speed for going to work and going home that day?

75 km/h 30 km/h 60 km/h 45 km/h Not enough information 90 km/h

Horace Pang by Horace Pang , 20, Hong Kong

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1 solution

Horace Pang
Jul 15, 2015

The average speed is defined as Total distance Total time {\text{Total distance}} \over {\text{Total time}} .

Let t t hours be the required time to drive to work. Therefore, the total distance is 2 ( 90 t ) = 180 t 2(90t) = 180t km. Since he went home with half the speed of going to work, he will need double the time to go home. So, the time required to go home is 2 t 2t hours. The total time is t + 2 t = 3 t t+2t=3t .

Hence, the required average speed is 180 t 3 t = 180 3 = 60 \frac { 180t }{ 3t } =\frac { 180 }{ 3 } =60 km.

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