Use a calculated (formula) field eg
{ =({ Mergefield totaldue } * 0.15) \# ",$0.00" }
Set the field boundary pairs with Ctrl+F9

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Graham Mayor - Word MVP
My web site www.gmayor.com
Word MVP web site http://word.mvps.org
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> Is it possible to apply mathmatical functions to a number inserted by
> mail merge.
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>
> Cory