Not knowing how to calculate data

[Beauty1]

I have been practising how to calculate a lot of data with excel formulas but I am still the dark. In most cases I am given a lot of data in different columns and asked to calculate for example how many customers who are different in classes bought a certain product in different countries. So my problem is that I do not know how to identify all those customers in different categories using a formula

[#2]

You would not use a formula for that ideally, but a pivot table. Alternatively, look at Help about the SUMIFS function, it will do what you require.