There are six letters a, b, c, d, e and f, according to the above method,

Maybe get AB first, then CD, and finally EF.

It is also possible to take the CD first, then the AB, and finally the EF, but the two are taken in a different order, but the result is the same, so we will repeat it. In this combination, a * * * has six situations, so the result of repetition is A3, so divide it by A3 and take 3.