Question

a boy runs for 10min at a uniform speed of 9km/h . at what speed should be run for next 20min so that average speed comes to 120 km/h.

Answer 1

HELLO THERE!

Total time that the boy runs for = 10 min + 20 min = 30 min

30 min = 30 x 60 = 1800 seconds.

For the first 600 seconds (10 min), he runs at a speed of 9km/h

9km/h = 9 x 5/18 m/s = 2.5m/s

So, distance covered = Speed x Time

= 2.5 m/s x 600 s

= 1500 m.

Now, given that, the average speed is 120 km/h

= 120 x 5/18 m/s = 100/3 m/s.

Average speed = Total distance/Total time

Let the total distance be x metres.

Therefore,

$\frac{100}{3} = \frac{x}{1800} \\or x = 60000m$

From this 60,000 metre, the boy has already covered 1500m.

Remaining distance = 60000 - 1500 = 58500m.

So, he has to travel with v speed to cover the rest

v = 58500/1200 (since time = 20 min = 1200s)

v = 48.75 m/s

On converting it to km/h,

48.75 m/s = 48.75 x 18/5 km/h = 877.5/5 kph = 175.5 km/h.

Answer 2

I hope my answer help you.