QUESTION: why we not assumed that all gas molecules have same speed as their collision is elstic
ANSWER: The fact that the collisions are elastic does not mean that the velocities are the same! Momentum must also be conserved and the masses of the particles are also different.
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QUESTION: if the mass of every particle is same then there is any transfer of kinetic energy between particles? we assumed that the gas molecules have same mass
In fact, the molecules will follow a "normal" type distribution with some particles moving very fast while others move very slowly with the majority moving near the center of the distribution. This is why water evaporates at room temperature where even though the temperature may be well below the boiling point, some of the molecules have much higher energies than the average and can leave the surface of the water even though the water is room temperature.