How many times 4 can go into 81?

Answer 1

4 can go into 81 twenty whole times. After that, the 81 is still not full, but there is only
enough room left in it for ' 1 ' more, so it is not possible to jam the entire 4 in again.

If you're allowed to cut the 4 up in pieces, then you can use ' 1 ' to fill up the rest
of the 81, and then unobtrusively discard the 3 that you have left over.