Question

A boy runs for 20 min.at a uniform speed of 18km/h.At what speed should he run for the next 40 min.So that the average speed comes 24km/hr.​

Answer 1

Explanation:

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Answer 2

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Lets divide the Journey into two parts.

Part 1

s = 18 km/h

t1 = 20 min. = 1/3 hr

∴ d1 = s * t1

⇒ d1 = 18 * 1/3

⇒ d1 = 18 / 3

⇒ d1 = 6 km

Part 2

s = Let 'x'

t2 = 40min. = 2/3 hr

∴ d2 = s * t2

⇒ d2 = x * 2/3

⇒ d2 = 2x/3

________________

Average Speed = Total Distance / Total Time taken

Total distance, d = d1 + d2

⇒ d = 6 + 2x/3

⇒ d = (18 + 2x) / 3

Total time, t = t1 + t2

⇒ t = 1/3 + 2/3

⇒ t = 1 hr

Average Speed = 24 km/hr (given)

∴ 24 = (18 + 2x) / 3*1           (just putting into formula)

⇒ 24 * 3 = 18 + 2x

⇒ 72 = 18 + 2x

⇒ 72 - 18 = 2x

⇒ 54 = 2x

⇒ 54/2 = x

⇒ 27 = x

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Speed for the second part = 27 km/hr

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