A man weighing 70 kg runs at a speed of 4 m / s, catches up with a cart weighing 45 kg, which moves

A man weighing 70 kg runs at a speed of 4 m / s, catches up with a cart weighing 45 kg, which moves at a speed of 3 m / s and jumps on it. How fast do they keep moving?

We use the law of conservation of momentum. In our case, it will look like this:
m1 * v1 + m2 * v2 = (m1 + m2) * u
u is the required speed of movement
u = (m1 * v1 + m2 * v2) / (m1 + m2)
u = (70 * 4 + 45 * 3) / (70 + 45)
u = (280 + 135) / 115
u = 415/115
u = 3.61m / s – Answer