That is, the unit 1 divided by the number of days they completed, then everyone's work efficiency is as follows: 1/20, 1/25,115.
When three people do it at the same time, their total efficiency is (1/20+1/25+115).
Reuse 47% of the completed project, divided by the total work efficiency, and get the number of days.
47%/( 1/20+ 1/25+ 1/ 15)=3
The principle is time = workload/work efficiency. The workload here is 47%, and the work efficiency is the sum of the work efficiency of three people.
You got it?