For a project, Party A will do it in five hours, and Party B will do it in three hours; Party B will do it in nine hours, and Party A will do it in three hours. How many hours does it take for Party A to do it in one hour?

For a project, Party A will do it in five hours, and Party B will do it in three hours; Party B will do it in nine hours, and Party A will do it in three hours. How many hours does it take for Party A to do it in one hour?

Because: a 3 hours + B 9 hours can also complete the work, so the remaining workload after a 1 hour is: (a 3 hours workload - a 1 hour workload) + B 9 hours workload, = a 2 hours workload + B 9 hours workload, = B 6 hours workload + B 9 hours workload, = B 15 hours workload A: then it will take 15 hours for Party A to do it one hour later

For a job, Party B can finish it in 3 hours after Party A does it in 5 hours; if Party B does it in 3 hours after Party A does it in 9 hours
When Party A does a job for 5 hours, Party B will do it for 3 hours; if Party B does it for 9 hours, Party A can do it for 3 hours, how many hours can party A and Party B work together?

5-3=2
9-3=6
That is, a 2-hour workload, B needs 6 hours
So the efficiency ratio of a and B is 6:2 = 3:1
Therefore, it takes 5 × 3 + 3 = 18 hours for Party B to do it alone
5 + 3 △ 3 = 6 hours
The cooperation between Party A and Party B needs 1 ÷ (1 / 18 + 1 / 6) = 1 ÷ 4 / 18 = 4.5 hours

For a job, Party A and Party B cooperate for 4 hours, and Party B and Party C cooperate for 5 hours. Now, after Party A and Party C cooperate for 2 hours, the remaining Party B needs 6 hours to complete, and how many hours does it take for Party B
The method is simple and easy to understand

A job, Party A and Party B cooperate for 4 hours, Party A and Party B's work efficiency is 1 / 4
B and C completed in 5 hours, B and C work efficiency 1 / 5
Now, after two hours of cooperation between Party A and Party C, the rest of Party B needs six hours to complete
（1-1/4*2+1/5*2）/2=（1-1/2+2/5）/2=20