First of all, the number of days originally planned to be used is calculated as 765÷( 18×3).

Then calculate the number of days in 765 (2 1×3), where 2 1 is the current population and 18+3 = 2 1.

Last Planned Days-Days Used Now = Days in Advance

Among them, the invariant is the number of episodes that everyone does every day, which becomes the number of people.