Question

How long will John take to make a round of a circular field of radius 21 m cycling at the speed of 8 km/hr?

Answer 1

We have:
The radius of the circular field = 21 m
∴ Circumference of the circular field = $2\mathrm{\pi }r=2\times \left(\frac{22}{7}\times 21\right)\mathrm{m}=132\mathrm{m}$.
If John cycles at the speed of 8 km/hr (In 1 hour John covers 8 km = 8000 m ), then,
John covers 8000 m in 1 hour.

​​∴ Time required to cover 132 m = $\frac{132}{8000}=0.0165\mathrm{hours}$
1 hour = 3600 seconds
∴ 0.0615 hours = 0.0615 x 3600 = 59.4 seconds.