DRDO Multi tasking staff Model paper 6

Let A be the faster than B and its speed be va and speed of B be vb
Now, when they move in the same direction, then velocity of A relative to B =vab=va−vb
And when they are moving in the opposite direction, then velocity of A relative to B =va+vb
Now, as per given,15/(va+vb) = 1/2
⇒va+vb=30
And 15/(va−vb) =2.5
⇒va−vb=

15

2.5

=

150

25

=6
⇒va−vb= 15/2.5 = 150/25 = 6
Adding the two equations obtained so far,
va+vb+va−vb=30+6=36
⇒2va=36
⇒va=18 km/hr