hello. this problem is just based on the concept of conservation of linear momentum. so when the man moves with a velocity,1m/s on a friction less troley the trolley should move backwards. here linear momentum of the man and the trolley should be conserved. hense we get velocity of trolley is0.25m/s. so in 4 sec man moves a distance 4m and trolley moves a distance 1m. so the resultant displacement of the man is 4-1=3m. this is ur required answer.