Question

the speed of a car is 72 km/hr . express it in m/s

Answer 1

1km/h=5/18m/s
72km/h=72×5/18m/s
=4×5m/s
=20m/s.

Answer 2

ANSWER WITH FULL EXPLANATION

If a speed is given $x$ km/h, then the formula to convert the speed from km/h to m/s is 
$x$ km/h = $(x*\frac{5}{18})$m/s

In other words, if you want to convert the given speed from km/h to m/s, then multiply the given number by $\frac{5}{18}$ and put "m/s" after it. 

The speed of a car is 72 km/h. 

Converting the speed into m/s
72 km/h = $(72*\frac{5}{18})$ m/s = 20 m/s

∴ The speed of the car (expressed in m/s) is 20 m/s. 

Hope you understood this very well. 

If you have any doubt, then you can ask me in the comments.