Who help me out 4 unknown unit "1" percentage application questions It's better to have an answer, or not

Who help me out 4 unknown unit "1" percentage application questions It's better to have an answer, or not

1. A project can be completed in 50 days by Party A alone, and 75 days by Party B alone. Now two people work together, but in the middle of the project, Party B leaves for a few days, and finishes the project 40 days after the start of the project. How many days did Party B leave in the middle of the project?
2. A pool can hold 84 tons of water. There are two water injection pipes to fill the pool. The first pipe can fill the pool in 8 hours, and the second pipe can fill the pool in 6 hours. Now, when the two pipes are opened at the same time and the pool is filled, how many tons of water can the second pipe fill the pool?
3. To build a road, team a will build it in 8 days, team B will build it in 10 days, team a will build it in 3 days, and the rest of team a and team B will build it together. How many days will it take?
4. Party A, Party B and Party C cooperate to produce a batch of machine parts. Half of the parts produced by Party A are equal to those produced by Party B, and three fourths of the parts produced by Party C. It is known that Party B produces 50 more parts than Party C. how many parts are there in total