Two bodies are moving with 20m/s in opposite directions. What is the relative velocity ?

Two electrons are moving in opposite directions with speeds 0.8c and 0.4c where c is the speed of lights in vacuum. Then the relative speed is about

Two bodies moving towards each other collide and move away in opposite directions. There is some rise of temperature of bodies in the process. Explain the reason for rise of temperature and state what type of collision it it ?

A shell of mass 20 kg moving horizontal at 4 m/s explodes into two parts. The part of mass 12 kg moves in the same direction as the shell with a velocity of 25 m/s. Find magnitude and direction of the other part.

A particle travelling along a straight line traversed one third of the total distance with velocity 4m//s . The remaining part of the distance was covered with a velocity 2m//s for half of the time and with velocity 6m//s for the other half of the time. What is the average velocity for the whole motion.