Why conservation of energy fails for this inelastic puck collision

[jackspicer]

Two pucks collide on a frictionless air-hockey table. Initally, puck A is traveling at 3.5 mls and puck B is at rest. After the collision, puck A moves away at a speed of 2.5 m/s and an angle of 30.0 from the initial direction. Find the final velocity of puck B. Puck A has a mass of 3.0 kg and puck B has a mass of 5.0 kg.

I know how to do the problem, but we can't we apply the conservation of energy methods because it is an elastic collision?

 

[Pengwuino]

In inelastic collisions you can't use conservation of energy.

 

[vivekrai]

He says that it is an elastic collision.