collisions and kinetic energy

Question

A 1 kg lump of clay moving through space at 2 m/s collides with a second 1 kg lump of clay at rest. if the lump sticks together what portion of the original kinetic energy is lost to internal energy?

 

Ans 50%

 

I know this is an inelastic collision and so kinetic energy is not conserved. I just don't understand how if the mass is doubled and velocity halved, how is kinetic energy halved? 

KE = 1/2 mv^2

KE = 1/2 (2m) (1/2 v)^2

KE = 1/2 (2m) (1/4v^2)

KE = 1/4 mv^2

Answer 1

You obtain a final KE of 1/4 mv^2

The initial Kinetic energy is 1/2 mv^2

Loss in KE = final - initial

Divide this loss in KE  by the initial KE and you will get the answer.