hang on a minute
If they have been painting for 100 years but the paint only lasts 40 years then the paint at one end of the bridge should have had the next repaint started 60 years ago surely?
If the bridge is 100 miles long and it took you a year to do a mile you'd only get 40 miles in before you had to start painting the first mile again.
So you get a second team in who start at the beginning and the first team does the next 40 miles.
The first team is now at 80 miles and the second team is at 40 so you need a third team to start the first mile again.
Now it gets tricky see because the first team can do the last 20 miles and the first 20 miles and the second team is at 80 and the third is at 40
I'll just get my coat and go now....