hi Bernadette,
Supposing your value is in A1, a formula field coded as {=A1*.2 \# 0.00} will give you 20% of that amount. To create the formula
field, press Ctrl-F9 to create a pair of field braces, type the formula in, then press F9 to update it. If you want the result
displayed as $, use {=A1*.2 \# $,0.00} - this will also insert a thousands separator.
Note that if you change the value in A1, you'll have to update the calculation. You can do this by selecting it and pressing F9.
Cheers