The master does two things a day, the apprentice does one thing in two days, and the apprentice does one thing in two days, that is to say, six days is a cycle, and the master and the apprentice do eight things every six days (in six days, the master does three days, which is six, and the apprentice does four days, which is two, which is eight): 2 * 3+ 1 * 2.

: 18+2=20 (days)