The link might look like a baffling equation (although it really isn't) but if you can't fully understand the maths, don't worry! Just remember this: where you get asked to input 'fractions' ensure you enter your figures as 'decimals', which means if your guess at one of the figures is 'one fifth' use a calculator to divide 1 by 5 (the answer to that would be '0.2') and that's the figure you need to enter. EASY!!! Go on, have a go... it's great fun!!!