Party A and Party B work together to build a section of highway. Party a works alone for 4 days to complete 13 days of this section of highway. Later, Party B joins two teams and works together for 3 days to complete the construction task of this section of highway A. 7 days B. 8 days C. 5 days D. 715 days

Party A and Party B work together to build a section of highway. Party a works alone for 4 days to complete 13 days of this section of highway. Later, Party B joins two teams and works together for 3 days to complete the construction task of this section of highway A. 7 days B. 8 days C. 5 days D. 715 days

1 △ [(1-13 − 13 △ 4 × 3) △ 3],, = 1 ± [(1-13 − 14) △ 3], = 1 ± [(23 − 14) △ 3], = 1 ± [512 △ 3], = 1 △ 536, = 715 (days), a: it takes 715 days for B to repair alone

An engineering team rush to repair a section of railway, the first team repaired 25%, the second team repaired 210 meters, and the two teams just repaired 40% of the total length?

210 (40% - 25%) = 210 (15%) = 1400 (m) a: this section of railway is 1400 m long

The two engineering teams of Party A and Party B rush to repair two sections of railway with the same distance at the same time. 12 days after the commencement, the workload completed by the two teams is equal to the total workload of Party A
20 days later, team B completed the task, and team a needed another 300 meters to complete the task

The workload of Party A and Party B is the same
12 days after the start of the project, the workload of the two teams is equal to the total workload of team a
Team B completed the task in 20 days
So the 8-day task of B is the 12 day task of A
So it takes 30 days for a to complete the task
That is, 300 meters will take 10 days
30 meters per day, so the total task is 30 × 30 = 900 meters
The length of two sections of railway is 900 × 2 = 1800 m