Alan: I'm sure I've answered this already! Have a look at: http://web2.0calc.com/questions/can-t-follow-reasoning
I'm truely sorry Alan, my wi-fi crashed when I uploaded it so I thought I'd lost it. Strangely enough I couldn't find it by using the search function either.
Thank you for your answer, I hadn't thought of dividing the two equation.
It is really helpful to me