How to get column total of quantity? i.e. This invoice has total x quantity of products

I want to know how to get Column total of quantity of products.
For eg consider invoice below

Pname quantity price linetotal
A 5 500 1500
B 4 200 800
C 6 200 1200

I want to show 5+4+6=15 in the invoice

how to do so? Please Help


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