Two cars start from the same place at the same time at right angels to each other . Their speeds are 54 km/h and 72 km/h, respectively .  After 20 seconds the distance between them will be ?

500 m

To convert km/h into m/sec . 

 X km/h  ×  \(\frac{5}{18}\) = Y m/sec 

54 km/h × \(\frac{5}{18}\) = 15 m/sec

72 km/h × \(\frac{5}{18}\) = 20 m/sec

Distance cover by both in 20 sec is  15 × 20  =  300 m , 20  × 20 = 400m

by using pythagoras theorem , we calculate distance between them after 20 sec. 

Distance between them = 500 m