Question

A and b run a 2 km race. A gives b a headstart of 100 m and still beats him by 20 seconds. If a runs at 20 km/hr, find b's speed (in km/hr).

Answer 1

time taken by a to complete the race
=2km÷20km/h
=1/10h=0.1h=6 mins= 360 s
thus time taken by b = 360+20=380 s
distance travelled by b = 2000-100
=1900m
speed of b= 1900m÷380s
=5m/s
=18km/h

Answer 2

A and b run a 2 km race. A gives b a headstart of 100 m and still beats him by 20 seconds. Then speed of B is 18 km/hr

Solution:

Given that,

A and b run a 2 km race

Distance = 2 km

A runs at 20 km/hr

Time taken by A to complete the race is:

$Time\ taken\ by\ A = \frac{distance}{speed } \\\\Time\ taken\ by\ A = \frac{2}{20} = \frac{1}{10} \hour \\\\Time\ taken\ by\ A = \frac{1}{10} \times 3600\ seconds \\\\Time\ taken\ by\ A = 360\ seconds$

A beats B by 20 seconds

Thus,

Time taken by B = 360 + 20 = 380 seconds

Find speed of B

Distance by B = 2 km - 100 m = 2000 m - 100 m = 1900 meter

Thus,

$speed = \frac{distance}{time} \\\\speed = \frac{1900}{380} \\\\speed = 5\ m/s\\\\speed = 5 \times 3.6\ km/hr\\\\speed = 18\ km/hr$

Thus speed of B is 18 km/hr

Learn more:

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