IB Physics SL A.2.20 Collisions and Conservation of Energy

B3. This question is in two parts. Part 1 is about a collision. Part 2 is about electric current and resistance.
Part 1 A collision
Two identical blocks of mass 0.17 kg and length 0.050 m are travelling towards each other along a straight line through their centres as shown below. Assume that the surface is frictionless.

The initial distance between the centres of the blocks is 0.900 m and both blocks are moving at a speed of $0.18 \mathrm{~m} \mathrm{~s}^{-1}$ relative to the surface.

As a result of the collision, the blocks reverse their direction of motion and travel at the same speed as each other. During the collision, 20 % of the kinetic energy of the blocks is given off as thermal energy to the surroundings.

State and explain whether the collision is elastic or inelastic.