Sum of total Quantity for a month - Urgent

Hi,

In 1 of the report we have to divide a particular key figure by the sum of quantity for a particular monnth - which will be provided by the user in selection screen.

If we view the data at various level the value of total quantity should not change. - How can we achieve this.

For e.g

deffect code no. of defects Quantity (this is the summation of quantity for given month)

101 10 350

102 20 350

103 17 350

how can we achieve the above output boldQUANTITY

Its very urgent.