Let's play mini-Sudoku!
We wish to place an "X" in four cells, such that there is exactly one "X" in each row, column, and 2x2 outlined box.
In how many ways can we do this?
(Terminology: A cell is one of small 1x1 spaces. A box is one of four outlined 2x2 group of cells.)
This was answered a while back, but the answer ends up being 16
4 ways to place the X in the top row
2 ways to place the X in the second row [ in the other 2 x 2 box not occupied by the first X ]
2 ways to place the X in the third row [ we can choose either of the two empty columns]
Only 1 way to place the last X in the last empty column
So ....4 x 2 x 2 x 1 = 16