Answer
We see that half of the kinetic energy was lost as a result of the collision.
Work Step by Step
We find the velocity of each of the masses after the collision, calling v the velocity of the moving mass before the collision:
$v_f = \frac{mv}{m+m}=.5v$
We compare the kinetic energies:
$k_0=\frac{1}{2}mv^2$
$k_f= \frac{1}{2}(2m)(.5v)^2 = \frac{1}{4}mv^2$
Thus, we see that half of the kinetic energy was lost as a result of the collision.
