Given 5 colors to choose from, how many ways can we color the four unit squares of a 2×2 board, given that two colorings are considered the same if one is a rotation of the other? A color can be used more than once
We have 5 options and must choose 4. However, the same color can be used more than once. 5*4 = 20, and (204) = 4845.
We did overcount (Example: 1234 is the same as 2341, but we counted it twice), so to take away overcounting, we divide 4845 by 5 to get 969. With some other casework and steps shown below, you will end up with 165.
Do not try to list out the answers because there are many and that is inefficient. Always use number theory.
- PM