# 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.?

2
by Prajakta123

2015-11-24T15:50:10+05:30
Distance covered in 10 minutes = 9 km 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
sry but ur answer is copied out frm net
2015-11-24T15:50:16+05:30
given ,
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...........................^_^

thanx frnd :)
Thanks Pahi and welcome too
:))