I Am In Class 9

Please Help Me With My Doubts

A car driver took a total of 2 hours to make a journey of 75 km. He had a coffee break of half an hour and spent a quarter of an hour stationery in a traffic jam. What was his average speed during the journey ?

Solution

The definition of AVERAGE SPEED is:

TOTAL distance traveled divided by the TOTAL time to travel that distance.

The TOTAL distance = 75 km

The TOTAL time = 2 h

therefore by definition the

AVG SPEED = 75/2 = 37.5 km/h

If the driver doesn t move for 1/2 h + 1/4 h = 3/4 h

then

she must make the TOTAL distance in 2 - 3/4 = 1 1/4 = 1.25 h

to do this, by definition, she must have an

AVG SPEED = 75/1.25 = 60 km/h

2) In the hare-tortoise race, the hare ran for 2 min at a speed of 7.5km/h, slept for 56 min and again ran for 2 min at a speed of 7.5km/h. Find the average speed of hare in the race.

total distance covered by the hare, s= 2(7.5*2/60) km = 0.5 km

total time taken, t = 60min = 1h

average speed, v = s/t = 0.5km/h

In both d questions we were asked to find average speed but in first question we did not included the time that he wasted or he was not moving and in second question we have included the time in which he was not moving

please tell which is correct

PRAKHAR GUPTA , 12 Years ago

Grade 9