A trolley weighing 20 kg moves at a speed of 1 m / s. It catches up with a trolley moving at a speed of 0.5 m / s

A trolley weighing 20 kg moves at a speed of 1 m / s. It catches up with a trolley moving at a speed of 0.5 m / s and engages with it, after which the trolleys move at 0.7 m / s. what is the mass of the second cart.

In this situation, we write down the law of conservation of momentum for a system of two carts:
m1V1 + m2V2 = (m1 + m2) * U
20 + 0.5m2 = (20 + m2) * 0.7
20 + 0.5m2 = 14 + 0.7m2
0.2m2 = 6
m2 = 6 / 0.2 = 6 * 5 = 30kg