Efficiency of a Spring

[LoveandHate]

Homework Statement

So, I have a 5.0 kg rat that goes down a ski slope, slides along a rough patch, and then smashes into a vertical board with a spring attached. I need to find out the efficiency of the spring with this information:
m=5.0 kg
v (after sliding across rough patch)=108 m/s ; it also says that because of internal friction, the velocity is 50% than above, so it's 54 m/s (after hitting the board and spring)
$$\Delta$$x=2.42 m (the magnitude of the spring's compression)

Homework Equations

e=e_out/e_in(100%)
Possibly E_k=.5mv² and E_elastic=.5k($$\Delta$$x)²

The Attempt at a Solution

Well, I think that E_in is equal to E_k, as this is the energy required for the spring to compress, but I'm not completely sure of E^out. Is it possibly E_elastic (I know this value from a previous question)? If I use these two though, I get an answer around 400%, which is obviously wrong.

 

[PhanthomJay]

Are you saying that it hits the spring with a velocity of 104m/s, then exits the spring with a velocity of 54m/s? It might be best to state the problem as exactly worded (both parts).