I don't see a chart, but, to find the mean you add all of the values up together and then divide by however many values you added.
For example, if she bakes 5 cakes one month, 7 cakes another month, and 2 cakes a final month, you'd add 5 + 7 + 2 to get 14. Then you'd divide 14 by 3, since you added 3 values together.
If you still need help, post the chart and I'll explain further.