Two particles one of mass m and the other of mass 2m are projected horizontally towards each other from the same level above the ground with velocities 10 m/s and 5m/s respectively. They collide in air and stick to each other. The distance from A, where the combined mass finally land is 

Relative acceleration between the two is zero therefore the relative motion between the two is uniform relative velocity of the two is (10+5) m/s or 15m/s in horizontal direction therefore the collision will take place after a  time
 $\text{t=}\frac{\text{60}}{\text{15}}\text{=4s}$
Net linear momentum in horizontal direction of the two particle is zero therefore after collision the combined mass will fall vertically downwards the desired distance fro A would be 40m.