Think about solving problems:
If the remaining expressway after the first phase is regarded as the unit of "1", then the second phase has already dug 2/7, and 5/7 has not been dug. At this time, the length of the excavated road is equal to the length of the excavated road, which means that the excavated road is also 5/7. According to the conditions, it can be known that two phases have been dug, the first phase is 180m, and the second phase is 2/7, that is, the first phase is 180m +2/7=5/7, and the score corresponding to 180m is 3/7, so it is divided by 3/7.
2. Take this project as the unit "1"
If the joint repair of Team A and Team B is completed in 10, then the joint repair of Team A and Team B in one day will be completed in110.
Two teams completed 4 *110 = 4/10 = 2/5 of the project in 4 days.
After 4 days of joint repair, the workload completed by Party A alone: 1-2/5=3/5.
It took Party A 20 days to repair this 3/5 alone, and it can be found that Party A needs to complete this project alone: 20/3/5= 100/3 (days).
Party A completes this project in one day:1100/3 = 3/100.
Party B completes this project within one day:110-3/100 = 7/100.
B This project needs to be completed alone:1/7/100 =100/7 (days).
3. Take this project as the unit "1"
Party A, Party B and Party C complete this project 1/30, 1/45, 1/90 in one day.
Party A, Party B and Party C cooperate to complete this project within one day:1/30+1/45+1/90 = 6/90 =1/5.
Party A, Party B and Party C need to cooperate to complete this project:1115 =15 (days).
Party A and Party C cooperate to complete this project within one day:1/30+1/90 = 4/90 = 2/45.
Party B and Party C cooperate to complete this project within one day:1/45+1/90 = 3/90 =1/30.
Party A shall rest for 2 days, and Party B and Party C shall cooperate for 2 days. Project completion1/30 * 2 =115.
Party B has a rest for 3 days, and Party A and Party C cooperate for 3 days to complete the project: 2/45*3=2/ 15.
The project beyond these five days is a joint project of Party A, Party B and Party C:1-115-2/15 = 4/5.
Four fifths of the engineering requirements of Party A, Party B and Party C: 15 * 4/5 = 12 (days).
It took 2+3+ 12= 17 (days) to complete this project.