4. In 2016, the world record for completing a 5000m three-legged race was 19 minutes and 6 seconds. It was set by Damian Thacker and Luke Symonds in Sheffield.
What was their approximate average speed in $\mathrm{km} / \mathrm{h}$ ?
A 10
B 12
C 15
D 18
E 25
Solution
C

The average speed running $5000 \mathrm{~m}$ in 19 minutes and 6 seconds is approximately the same as running this distance in 20 minutes. So their average speed was approximately $\frac{5000}{20}$ metres per minute, that is 250 metres per minute.

A speed of 250 metres per minute is the same as $60 \times 250$ metres per hour, that is, 15000 metres per hour. This is the same as $15 \mathrm{~km} / \mathrm{h}$.