Could somebody help ? A boy with mass 5okg running at 5m/s jumps on to a 20kg trolley traveling in the same direction at 1.5m/s. What is their common velocity ?!

Ahot-air balloon takes off from the ground traveling vertically with a constant upward acceleration of magnitude g/4. after time interval δt, a crew member releases a ballast sandbag from the basket attached to the balloon. how many seconds does it take the sandbag to reach the ground? (express your answer in terms of δt)