A 20 kg projectile flying horizontally hits a platform with sand of mass 10^4 kg and gets stuck in the sand. With what speed did the projectile fly if the platform started moving at a speed of 1 m/s?

A detailed solution with a brief note of the condition, formulas and laws used in the solution, the derivation of the calculation formula and the answer.
If you have any questions about the solution, write. I try to help.