Question 403876
The relationship between Distance, Speed and Time is found in this formula:


D=ST


You have D at T, and you're looking for S. First, let's change T from 30 minutes to 0.5 hours, so that our speeed will be in km/h.


D=ST
21 = S(0.5)
S = 21/0.5
S = 42 km/h


So the average speed is 42 km/h.


Hope this helps!