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 ?!

Why does karst topography exist in some geographic location? a. few climates have enough oxygen b. most regions have rock layers other than limestone c. in most regions, the soil is too thin d. the water is too acidic enough in most places