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Charlie's number:

We know from clues C2 and C5 that Charlie's number is somewhere between 100-500.

It is palindromic, so the first number and the last number must be the same, but the middle number is different.

Because it is an odd number, the last digit cannot be 2 or 4. This leaves 1 or 3.

If the first and last digits were both 1, then the middle digit would have to be 12 for them to add up to 14, which isn't allowed. Therefore the first and last digit must be 3 and the middle digit must be 8.

Just to check,383 is a prime number and the difference between 3 and 8 is 5, so this is definitely the right answer.

Tower of cubes:

We deduce that there are 2 blue blocks, 2 yellow blocks, 1 red block and 1 green block. One of the yellow blocks is at the top, so we can put that one in place.

In order for both of the blue blocks to be in contact with the green block, they have to be either side of it, directly above and below. That means that for the red block to be higher than the green block, it must also be above one of the blue blocks, and for the other yellow block to be below the green block, it must be right at the bottom.

The resulting tower looks like this: