How to automate calculation in MS Word and Zoho Writer? | Sum, Product and many more

In this video, I show how you can automate some of the standard calculations in MS Word.
If you are creating simple invoices or purchase orders and you need to total the items or multiply the quantity by price in a word document you don't need to rush to MS Excel or search the calculator. Word can do it for you automatically.
In this video I show how you can do it using MS Word and Zoho Writer. As of now, Google Docs or Open Office perhaps do not have this support.