The wheel of a motorcycle 70cm in diameter makes 40 revolutions in every 10sec.What is the speed of motorcycle n km/hr?

In this type of question, we will first calculate the distance covered in given time.

Distance covered will be : Number of revolutions x Circumference 

So we will be having distance and time, from which we can calculate the speed. 

Radius of wheel = 70 / 2 = 35 cm

Distance covered in 40 revolutions will be  $40*circumferene=40*2\mathrm{\pi r}=40*2*\frac{22}{7}*35=8800\mathrm{cm}$ = 88 m

Distance covered in 1 sec  =  $\frac{88}{10}=8.8m/s=8.8*\frac{18}{5}km/hr=31.68km/hr$