The mathematical method should be:

Guaranteed not to carry, thousands can only be selected from three numbers of 0, 1, 2, hundreds and tens can only be selected from 0, and units can be selected from six numbers of 0, 1, 2, 3, 4 and 5.

So a * * has c (1, 3) * c (1, 6)/2 = 3 * 6/2 = 9 options. (Divided by 2 because 0,5 and 5,0, 1, 4 and 4, 1.

(C( 1, 3) means that if I choose one of the three, I can't type this mathematical symbol.)