Two particles moving toward each other (in the same dimension), A and B. A has a mass of 3 tonnes (3,000kg) and B, a mass of 1 tonne (1,000kg). A is moving at 3m/s, B's speed is unspecified. The two particles collide to form a single particle, C, mass 4 tonnes (4,000kg).

What is the speed and direction of C?


How can I calculate its speed without knowing the initial momentum of B?

You needn't worry yourselves. I found the marking scheme to this problem; and, from studying the steps of the working, it looks like they include an initial velocity for B which they forgot to include in the question.