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