Thank you very much for your help.
I am okay with the equation, my problem is when I have to work out with numbers and I do not spot the logic that is in them. When you both explained to me, I understood it quickly, I am practising loads of examples and I can solve most of them, but from time to time I get stucked! My background is Socials Science....however, I will keep working hard.
The funny point will come when I start timming myself!!!
Well guys, thanks again. You can not imagine how much you help me.