# A boy runs for 10 min at a uniform speed of 9 km/hr.At what speed should he run for next 20 min so that the average speed comes 12 km/hr.?

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by Prajakta123

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by Prajakta123

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Speed for the 2nd leg = V kmph

Distance coveed in 2nd leg = V * 20/60 = V / 3 km

Overall average speed = total distance / total time =

( 1.5 + V/3 ) / (10+20)/60 = 12 km ph

2 (1.5 + V/3) = 12

1.5 + V/3 = 6

V = 13.5 KMPH

The Brainliest Answer!

time1 = 10 min = 1/6 h

time2 = 20 min = 1/3 h

speed1 = 9 km/h

speed2 = x km/h

distance1 = s * t = 1/6 * 9 =1.5 km

distance2 = s * t = x * 1/3 = 1/3x

average speed = 12 km/h

therefore ,

av.velocity = total displacement / total time

12 = (1.5 + 1x/3 ) / 1/6 + 1/3

12 = ( 15/10 + 1x/3 ) / 1/2

12 = ( 15/10 + 1x/3 ) 2

12 = 30 / 10 + 2x / 3

12 = 3 + 2x/3

12 - 3 = 2x/3

9 = 2x/3

9 * 3 = 2 x

27 / 2 = x

13.5 = x

therefore the answer is 13.5 km/h .

i hope it helps...........................^_^