Calculating averages

[mr_teacher]

Hi,

I am wanting to create an average score for the mental maths tests of the children in my class.

i am fine at getting the average for all the tests to date - but the part I am struggling with is that i would like to see if their average has improved from the previous test. For example ifthe children had just taken Test 17 I would like to know if their average is better or worse than their average was by Test 16. I want the average score to be over all tests up to that point.

I have included the beginnings of the sheet that will hopefully make a little bit more sense?

Thanks for the help

Carl