The radius of the wheel of a vehicle is 42 cm.

Radius of the wheel = 42 cm

Circumference of the wheel $=2 \pi \mathrm{r}$

$=2 \times \frac{22}{7} \times 42$

$=264 \mathrm{~cm}$

$=\frac{264}{100} \mathrm{~m}$

$=2.64 \mathrm{~m}$

Distance covered by the wheel in 1 revolution = 2.64 m

Total distance = 19.8 km or 19800 m

$\therefore$ Number of revolutions taken by the wheel $=\frac{19800}{2.64}=7500$