Excell 2010 Formula Query

I'm a little stuck with a sheet I'm creating at work.

Basically the sheet tracks performance across 3 fields for our suppliers. Delivery, Product Quality and Supporting Documentation.

I would like to do an average sum at the bottom of each of these fields, then an overall average.

However the averages won't work unless every box has been populated, and as it's a continual assessment, this isn't possible, how can I make it average the cells that are populated and ignore the ones the arent?

Much appreciated for the help.

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