Extracting VAT from a cell or group of cells

If your VAT is already being included with the amount, you can simple use this formula which gives you the amount of VAT:
(20/120)*Gross amount. Gross amount is inclusive of VAT. Or simple divide 20/120=0.166666667 and then multiply it by the gross figure.

Thanks so much for the replies, unfortunately it has highlighted that as suspected I did NOT give you enough information. I will now try to explain proporly.

Cell A1 Has amounts from i.e. Fuel bills for May 2012, which not all these bills are subject to 20% VAT, they may also be vatable at a rate of 5%. Is there anyway which I can differentiate as I am inputting how much VAT is within the Gross figure? THEN transfer the VAT amount to another column. (I suspect that I mihgt just have to do a VAT column and set up the basic SUM formulae and just add to it each month.

Or anyone who is accounts trained, do you have a way when your doing books/accounts where you differentiate within a workbook?