Break simplified DES
Here's a hint on one avenue of approach (actually, there are several possible):

Suppose we had two plaintexts $X$ and $X'$, which are identical except in one bit.  Now, consider $\text{SimpleDES}_K(X)$ and $\text{SimpleDES}_K(X')$; how are they related?

If that's too difficult, consider how both $\text{SimpleDES}_K(X)$ and $\text{SimpleDES}_K(X')$ are related after 1 round, after 2 rounds, etc...