A CAR MAKES 50,000 REVOLUTIONS TO TRAVEL A DISTANCE OF 121KM.THE DIAMETER OF THE CAR IS

Please check your question once, it should be diameter of wheel of the car instead of diameter of car.

Number of revolutions = 50,000
Distance covered = 121 km = 121,000 m
So, circumference of wheel of a car = $\frac{\mathrm{Distance} \mathrm{covered}}{\mathrm{number} \mathrm{of} \mathrm{revolutions}} = \frac{121000}{50000} = 2.42 \mathrm{m}$
Suppose r be the radius of wheel of car.
So, circumference of wheel of a car = $2\mathrm{\pi r} = 2.42$
$\Rightarrow r = \frac{2.42}{2\mathrm{\pi }} = \frac{2.42}{2\times {\displaystyle \frac{22}{7}}} = \frac{2.42\times 7}{2\times 22} = 0.385$
so, the diameter of wheel of the car = 2r = $2\times 0.385$ = 0.77 m