S1 venn diagrams

If I wanted to fill out the space where it is A and C but not B normally you'd do 100 - 10, no?

No

Because it said A and C but not B = 100

It actually gave you that section

It said NOT C

So if it said NOT x then you just put the value in? And if it just says A and B without mentioning C you would've minused the intersection first?

Yes

I think you're approaching this mechanically rather than understanding what each sector in the Venn Diagram means. For this question, there are three sets A, B, C. You've understood where the three sets intersect (A and B and C). You don't seem to have understood that there are three further intersecting sectors. It's pretty obvious from the questions what these are.

Your teacher is correct in that normally you have to calculate the value for every sector in questions of this type, but in this case they're mostly given. The only one you have to calculate is the one outside the sets.

Hope this helps.