Question

A car moves with a speed of 40 km/h for the first hour and then with a speed of 60 km/h for the next half hour and finally with a speed of 30 km/h for the next 1.5 hours. What will be the average speed of the car?

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• A. 39 km/h
• B. 35 km/h
• C. 37 km/h
• D. 40 km/h

Answer 1

The correct option is B 35 km/h

First part of motion:
Time, $t_1=1h$
Speed, $v_1=40km/h$
Distance travelled:
$s_1=v_{1}t$
$s_1=40km/h\times 1h$
$s_1=40km$

Second part of motion:
Time, $t_2=0.5h$
Speed, $v_2=60km/h$
Distance travelled:
$s_2=v_{2}t$
$s_2=60km/h\times 0.5h$
$s_2=30km$

Third part of motion:
Time, $t_3=1.5h$
Speed, $v_3=30km/h$
Distance travelled:
$s_3=v_{3}t$
$s_3=30km/h\times 1.5h$
$s_3=45km$

Total distance:
$s=s_1+s_2+s_3$
$s=40km+30km+45km$
$s=115km$

Total time taken:
$t=t_1+t_2+t_3$
$t=1h+0.5h+1.5h$
$t=3h$

Average speed:
$v=\frac{s}{t}$
$v=\frac{115km}{3h}$
$v=38.3km/h$