This is not for math nerds, this is very simple for those who did not forget math courses from high school
And if you forgot it is still very simple to count "manually" and see the patern for the formula, like this:
1 shoe 1 shirt, 1 pant, 1 watch
1 shoe 2 shirt, 1 pant, 1 watch
1 shoe 3 shirt, 1 pant, 1 watch
........................................
1 shoe 12 shirt, 1 pant, 1 watch
1 shoe 12 shirt, 2 pant, 1 watch
1 shoe 12 shirt, 3 pant, 1 watch
..........................................