How do you solve this
Each unit square of the 4 x 4 grid below is to be filled in with the number 1, 2, 3 or 4 so that
* each row contains the numbers 1, 2, 3, and 4 in some order
* each column contains the numbers 1, 2, 3 and 4 in some order
* each 2 x 2 square (outlined in bold) contains the numbers 1, 2, 3, and 4.
Some of the square have already been filled in. Find the number of ways of filling in the rest of the grid.
