I've just been doing the official practise DM test and I'm a bit confused on a Venn Diagram explanation. The questions states that "There are 150 aquariums, 60 have yellow crabs, 81 have starfish and 43 have white dolphins, 20 have both starfish and white dolphins, 12 have both white dolphins and yellow crabs and 9 have both starfish and yellow crabs and 7 have all 3. They have drawn a Venn diagram with 7 in the middle, which I understand, but then on the overlaps, between white dolphins and yellow crabs for example, they have put in 12 and not taken away the 7 in the middle. I thought that for this, you would do "12-7" because out of the 12 that have both dolphins and yellow crabs, 7 would have all 3? I dont know if this makes sense and can put in a picture of the diagram they have drawn if it makes it any easier! Any ideas?