How to add coloumn at run time in the crystal reports????

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C#2.0

C#

Hello,
I am using dot net frameworks crystal reports.I am designing report into which i have to add a columns related to Tax Percentage.In brief...I have number of products which have variable tax discount percentage..suppose Product A,B,C,D have 5% Tax,Product E,F have 12.5 % tax.The tax % may be vary.
so what i m trying to do is for a certain time period i have to list total amount of tax as per Tax percentage...Suppose for time period 1/12 to 15/12
products with vat 5%,12.5% and 20% have sold then in my report i have to display 3 coloumns with heading 5,12.5,20 and there amount in details.The Tax Percentage will be vary for certain time period.So according to Tax percentage no of column also vary...How TO do this???