100*6=600 ... 6 days in advance. If the planned number of days is maintained, it can be done for another 600 days.
100-80=20。 . . . Actually, I do 20 more than planned every day.
600/20=30。 . . . Make 20 more every day. If the planned number of days is reached and * * * makes 600 more, then the planned number of days is 30 days.