A boy runs for 10 min at a uniform speed of 9 km/h. At what speed should he run for the next 20 min so that the average speed comes to 12 km/h?
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Explanation:
9 km/hr = 9×1000/3600 m/s
= 2.5 m/sec
t2 = 20 min
v = 12km /h
= 12 × 1000/ 36000 m/sec
= 3.33 m/sec
d = 2.5× 600
= 1500 m
Average Speed = 2xy/ x+y
= 2×2.5× y/ (2.5 +y)
3.33 = 5y/ 2.5 +y
8.325 + 3.33 = 5y
1.67 y = 8.325
y = 8.325 / 1.67
= 4.98 m/s
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