A cube has six numbers marked 1,2,3,4,5, and 6 on its faces. Four views of the cube are shown below :

What is the number on the face opposite the face showing 4?

1. Views 1 and 2: The numbers 5 and 6 are common to both views. The other visible numbers are 1 and 3. This means that 1 is opposite to 3.

2. Views 2 and 3: The numbers 3 and 5 are common to both views. The other visible numbers are 6 and 2. This means that 6 is opposite to 2.

Now, A standard cube has six faces. We have already found two pairs of opposite faces: (1, 3) and (2, 6). The only remaining numbers are 4 and 5. Therefore, the last pair of opposite faces must be 4 and 5.