Question

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

• A. 12.5 km/h
• B. 10 km/h
• C. 13.5 km/h
• D. 14.5 km/h

Answer 1

The correct option is C

13.5 km/h

We know that,

Average speed * Total time = Total distance

Here total time = 10 mins + 20 mins = 30 mins

So, total distance = 12 km/h * $\frac{30}{60}$ h = 6 km

Distance covered in first 10 mins = 9 km/h * $\frac{10}{60}$ h = 1.5 km

Hence, speed for rest of the journey = $\frac{(6-1.5)km}{\left(\frac{20}{60}h\right)}$ = 13.5 km/h