You use mixed cell references when you want to copy your formula down and across and you only want to write one formula.
If you have percentages in cells B1 to D1 and you have a series of numbers in A2 to A10. You could insert the following formula in cell B2 and copy it down to B10 and then copy across D10 which will multiply the percentages in row 1 by the figures in column A
=B$1*$A2.
You fix row 1 with $ and column A with a $ and you only need to write one formula and then copy it.
You can also use range names to achive a similar affect.
HTH
