Descriptive Statstics Question!!

Hey guys,

I need your help:

I got following question:

Six customers have Tested a new chocolate bar an have rated it on 0-10 scale (10=best) . Calculate the average rating based on how often the customers go shopping and how many chocolate bars they usually buy each time they go shopping.

rating: 8 4 5 7 10 8
shopping/month: 6 12 4 4 6 10
Chocolate bars. 1 1 2 3 1 1

I know the answer is 6.67 but I don't get it. Do I have to get a specific mean and how do I put both data in relation to each other?:rolleyes:

Thank you for your help.