To solve the problem you must first know that by keeping the linear moment P1 = P2. You must find P1 from the system and equal it to P2 of the system, from that equation you clear the final velocity 1. Which will result in V1f = 60.16 cm / s to the north.I attach the solution.
She can put chalk in vinegar as the vinegar will disintegrate the chalk chemically demonstrating chemical changes. But for physical changes she can break the chalk into small pieces by smashing it with something or her hand.
