How to get the Average value in the report.

Hi All,

I have a requirement to get the average value of the Key figure like below.

1.If we enter the Fiscal year period for one year(01.01.2012 - 31.12.2012)(12 months) in variable screen the value of the Key figure in the report should calculates as below.

200/12=16.66  (considering KF value =200 ).

If we  enter three months in the selection screen then the denominator of the formula should be three months like

200/3=66.66

Would you please help me how to achieve this in report level.