C42 is to select two of the four balls, C2 1 to select 1 from the remaining two balls, and the reason for dividing by A22 is to cancel the arrangement order of the two separately selected balls, because all the balls and boxes are arranged behind A43.

This problem only needs C42*A43, which is redundant.

Because there is an empty box, it means that four balls are put in three boxes, only 1, 1, 2, so the two balls should be tied together to form a whole.

In addition, A43 means 1, 1, 2. Choose three from the four boxes and arrange them all. Divided by A22, the complete arrangement of 1 and 1 is redundant.