Question

two cyclist start from the same place in opposite direction one goes towards north at 18 km per hour and the other goes towards south at 20 km per hour what time will they take to be 47.5 km apart ?

Answer 1

relative speed = 18+20=38
time=distance/speed
=47.5/38= 5/4hr
= 1.25 hr

Answer 2

t = 1.25 hr

We can do this question by relative motion

Lets consider A going to north(+ve) and B going to south(-ve)

so Speed of A w.r.t B= S of A - S of B

=18 - (-20)

Therefor Relative Speed= 38 km/hr

now s= u.t

47.5=38.t

t=1.25 hr

Hope you Understood