Question 1023896
Here is what I think you mean here:
 In a car the front & rear wheel diameter is 21 & 24 cm.
 Find the no. of revolutions that the rear wheel makes to cover the distance which front wheel covers in 2 sec at a speed of 252km per hour 
:
Find the circumference of both wheels:
{{{pi*21}}} = 65.973 cm
{{{pi*24}}} = 75.398 cm
:
Find the distance in meters traveled in 2 sec at 252 km/hr
2 * {{{(252*1000)/3600}}} = 140 meters
Change to cm: 140*100 = 14000 cm
:
Find the no. of rev the rear wheel makes to travel 14000 cm
{{{14000/75.398}}} = 185.7 rev
During this time the front wheel travels the same dist but makes
{{{14000/21}}} = 212.2 rev
:
note that the ratio of the rev of the two wheels
185.7/212.2 = .875 the same as 21/24 = .875