Question

An airplane flies 21 km in 30 minutes. What is its average speed in kilometers per hour?

Answer 1

Average speed = (distance covered) / (time to cover the distance)

Average speed = (21 km) / (1/2 hour)

Average speed = (21/0.5) (km/hour)

Average speed = 42 km/hr

A very slow airplane !  Maybe an ultralight, a motorized hang-glider, etc.

Answer 2

30 minutes is 1/2 of an hour.

So if you just multiplied 21×2, you would get you answer.
The answer is:
42 kilometers per hour