Question 27429
When working together, crew A and B can lay cable at a rate of 4 miles per day (2 miles each).

Now, Crew A has already been working alone (rate: 2 miles per day) for 6 days. So there are already 12 miles of cable laid. Since the total is 40 miles, there are still 28 miles to go. But now crew B starts working too, so the rate goes to 4 miles per day. Since 28 miles are left, then they must work for 28/4 = 7 days more.