Explanation:

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*2πr=40*2*22/7*35=8800cm = 88 m
Distance covered in 1 sec  = 88/10=8.8m/s=8.8*185km/hr=31.68km/hr