If I have four boxes arranged in a 2 x 2 grid, in how many distinct ways can I place the digits 1,2 ,3 and in the boxes such that each box contains at most one digit? (I only have one of each digit, so one box will remain blank.)
We can place a 0 in the fourth box that will be empty.
If we know this, the problem becomes very simple. There are fourr choices of digits for the first box, three for the second, two for the third, and one for the last. 4*3*2*1=24 total options.