1. Video - Formula method - http://www.youtube.com/profile?user=jaslocombe#p/u/29/Y22_AC6BN9Y
2.Video - Chart method - http://www.youtube.com/profile?user=jaslocombe#p/u/30/TLOo2mY6FIw
3. Video - Quick chart method - http://www.youtube.com/user/pajholden#p/u/70/fwrCAPuR4Yo
4. BBC Bitesize - http://www.bbc.co.uk/schools/gcsebitesize/business/finance/profitabilityrev1.shtml
