10% off the number i type into it? i have made up an invoice and this would save alot of time.

You can't have both a formula and a data point within the same cell. One or the other. You'll need a second cell that calculates the 10% deduction from the value entered into the first cell. The formula should look something like this:

B1 = A1 * (1 - .1)
or
B1 = A1 * 0.9

Well, not without a macro, but what about a second cell that takes the value of the first cell and subtracts 10%. To do this, suppose the value you enter is in c1. Enter a formula of

=c1*.9

into cell d1 to get the 10% discounted value.

The only way I know to do this is to type it into another cell (maybe on another sheet) and then use a formula in the cell that you need the lower number in...i.e. =sheet2!A1-(sheet2!A1*.1) You may also want to add conditional formating to make the font color match the background color if the cell = 0...that way you don't have a bunch of 0s uglying up your Spreadsheet.

no, but it can be in the cell next to it, or anywhere else for that matter. =A1*0.9 is the code where A1 is the cell that you're entering data into.

Yes. When you enter the number you must enter it as a formula. Very basic formula.

=1.10*.9

where 1.10 is your amount and *.9 is the deduction. It will automatically calculate 90% of the amount you enter and subsequently will be the 10% deduction your are looking for. Format the cell to display as dollars or accounting dependent on your invoice style.