# 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 12km/hr

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Distance covered in 10 minutes = 9 kmph * 10/60 hrs = 1.5 km

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

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

Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.

So t = 60 min=1/6 hr

speed = 9 km/h

displacement = 9 x 1/6 = 1.5 km

again

let speed = X km/h

time = 20 min = 20 / 60 = 1/3 hr

displacement = X x 1/3 = X/3

so av. velocity = total displacement /time taken

=

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

⇒x = 13.5 km

speed = 9 km/h

displacement = 9 x 1/6 = 1.5 km

again

let speed = X km/h

time = 20 min = 20 / 60 = 1/3 hr

displacement = X x 1/3 = X/3

so av. velocity = total displacement /time taken

=

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

⇒x = 13.5 km