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

given

Average speed = total distance / total time,

Distance travelled in 20 min = 18/3 = 6 km,

Let the distance travelled in 40 min be x km ,

(20 min = 1/3 of an hour, 40 min = 2/3 of an hour)

Then : 24 =( x + 6)/(2/3 + 1/3), = x+6/1,

X + 6 = 24km, x = 18km , now let the speed required be y, then we know that y = 18/(2/3)

( speed = distance/time) so y = 18 *3/2 km/hr

Y = 27 km/hr

hope you understood.

Answer 2

Given,

°Time(t1) = 20minutes

=>Time(t1) =20/60 hours = 1/3 hours

Speed(S1)= 18km/hr

Hence, distance= 18×(1/3) = 6km

° Time(t2)= 40minutes.

=>Time(t2) = 40/60hours = 2/3hours

let the speed be X km per hour.

So, distance= 2x/3 km

Final speed(S2) = 24km/hr

Now, According to the question =>

Final speed = Total distance/total time

=>Final speed = {(6 + 2x)/3}/(2/3+1/3)

=>24 = 6+ (2x/3)

=> 2x = 54

=>x = 27km/h.

Hence the speed of the boy is 27km/h..