Two parallel rail lines are directed as north- south. A train X runs towards north with a speed of `15m*s^(-1)` and another train Y runs towards south with a speed of `25 m*s^(-1)` .
(iii) velocity of a monkey running on the roof of X against its motion with a velocity of `5 m*s^(-1)` relative to X, as observed by a man standing on the ground.

Explanation

Let us assume the velocity from south to north be positive. Then,
Velocity of x, `v_(X)=+15m*s^(-1)`
and velocity of Y, `v_(Y)=-25m*s^(-1)`
(III) The relative velocity of the monkey with respect to train X is,
`v_(MX)= v_(M)-v_(X)=-5m*s^(-1)`
(where, `v_(M)` is the velocity of the monkey with respect to ground )
`therefore v_(M)=v_(X)+v_(MX)=15+(-5)=10m*s^(-1)`