The type of collision in which the object stick with one another is a perfectly inelastic one. For inelastic collisions, the momentum is preserved such that,
M1 = M2
Momentum is the product of the objects mass and velocity. In this item, we are given that the after sticking with one another, the trucks moved at a speed half of the speed of the originally moving one. Hence, the trucks must have the same load which is equal to saying that they have equal masses.
