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

2
by himanshu9

2015-10-30T12:38:21+05:30

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Speed = 9 km/hr
Time = 10 min = 10/60 hr = 1/6 hr
Distance = Speed × Time = 9 km/hr × 1/6 hr = 3/2 km

Let speed for next 20 min = x km/hr
Time = 20 min = 20/60 hr = 1/3 hr
Distance = x km/hr × 1/3 hr = x/3 km

Average Speed = Total Distance / Total Time
⇒12 km/hr = (3/2 km + x/3 km) / (1/6 hr + 1/3 hr)
⇒ x = 13.5 km/hr
So, the boy has to travel at speed of 13.5 km/hr for 20 min .

No my answer is cent percent correct!!
Right answer*** by mistake wrotw wrong
Not totally wrong just last step wrong.
2015-10-30T20:53:24+05:30
Speed =9 km/hr                                               time=10 min
distance=9000m/6=1500m
speed=12km/hr                                                             time=30min
distance=12000m/2=6000m
distance6000-1500=4500m                                 time=20min
speed=4500/3=1500m/hr or15km/hr

He should run at 15km/hr so that average speed comes to 12/h