Fred, Joe and Bill do plumbing, electrics and plastering respectively and work together on certain building jobs. They will each take on any job involving all three types of work, sub-contract to the others where necessary and sort out any payments at the end of the month. At the end of March, Bill has done £300 of work for Fred and £200 of work for Joe. Joe has done £100 of work for Fred and £250 of work for Bill. Fred has done £150 of work for Joe and £200 of work for Bill.