Question

An object of mass 80kg moving with velocity 2m/s hit by collideswith another object of mass 20kg moving with velocity 4m/s find loss if energy assuming a perfectly inelastic collision

Answer 1

Were they going in the same or different directions?
In either case, momentum before = momentum after.
If collision is inelastic the stick together so use 100 kg for the mass on the right hand side.

Answer 2

For an elastic collision collision the total kinetic energy is conserved, when the collision is inelastic some of the kinetic energy is lost from the system while in a perfectly inelastic collision the two objects will stick together after the collision and move off together.

So, you need each objects initial mass and velocity, and the final velocity of the coMingling mass. Something is missing in your description.
energy lost= sum initial KEnergies-final KE energy

Answer 3

Given:
M1 = 80kg, V1 = 2m/s.
M2 = 20kg, V2 = -4m/s.

V3 = Velocity of M1and M2 after collision.

Momentum before = Momentum after.
M1*V1 + M2*V2 = M1*V3 + M2*V3,
80*2 - 20*4 = 80*V3 + 20*V3,
100V3 = 80.
V3 = 0.80 m/s.

KEb = 0.5M1*V1^2 + 0.5M2*V2^2 = 40*4 +10*16 = 320 J. = KE before collision.
KEa = 40*0.8^2 + 10*0.8^2 = 32 J. = KE after collision.
KEb - KEa = 320 - 32 = 288 J. = KE lost.