Because there is at least one letter in each mailbox, the situation after putting it away should be: there are two letters in 1 mailbox and one letter in each of the other two mailboxes. In other words, four emails are divided into 2- 1- 1. C42 in front (I can't type it out, you know what I mean by C42) means that two of the four emails are selected as a group, and the other two are each a group, thus dividing the four emails into three groups, 2- 1- 1. Oh, that's great. C42=6

Then it is distributed after grouping, that is, deciding which mailbox to put each group of mail in. Three groups of mails, three mailboxes, obviously all three are full, that is, the last item is 33 (it seems to be a density symbol ... strange), which is generally used to write A33=6.

The result is the superposition of all grouping and allocation situations, 6*6=36.

Start (a disease)