An athlete runs 8 times around a circular field of radius 7 m in 3 minutes 40 seconds. His speed (in km/h) is:

$$\eqalign{ & {\text{Time}} = 3\,\min 40\sec \cr & = 3\frac{{40}}{{60}} \cr & = 3\frac{2}{3} \cr & = \frac{{11}}{3}\min \cr & {\text{Circumference of circle}} = 2\pi r \cr & = 2 \times \frac{{22}}{7} \times 7 \cr & = 44{\text{ m}} \cr & {\text{Distance}} = {\text{Speed}} \times {\text{Time}} \cr & {\text{44}} \times {\text{8}} = {\text{Speed}} \times \frac{{11}}{3} \times 60 \cr & {\text{Speed}} = \frac{{32}}{{20}} \cr & {\text{Speed}} = \frac{8}{5}\,{\text{m/s}} \cr & {\text{Speed}} = \frac{8}{5} \times \frac{{18}}{5}\,{\text{km/hr}} \cr & {\text{Speed}} = \frac{{144}}{{25}}\,{\text{km/hr}} \cr} $$