The way to solve this is to find the time it took for each lap, add those 4 numbers and divide the sum into the length of 4 laps.

Let’s say the distance of one lap is ‘d’ kilometres.

The time it took to complete the lap at 12 km/h is d/12; the time for the 2nd lap is d/16; for the 3rd lap d/15; and for the 4th lap d/10.

Now if we add these 4 times we get d/12 + d/16 + d/15 + d/10 =

(20d + 15d + 16d + 24d)/240 = 75d/240 = 5d/16 (the total of the 4 ‘heats’)

So the average speed (distance/time) is 4d/(5d/16) = 64/5 km/h = 12.8 km/h