Why does finding the union of these three sets yield a negative number?
I've been working on a homework problem that I can't seem to be able to solve.
The question states:
Suppose 25 people attended a conference that contains 3 sessions. 15 people attended session #1; 18 people attended session #2; 12 people attended session #3. At least how many people attended all 3?
However, when I'm applying this problem to 3 sets, I obtained a negative number.
I computed the result: |A∪B∪C| ≤ 5 where |A∩B| ≥ 8, |A∩C| ≥ 2, |B∩C| ≥ 5, however, I don't quite understand the negative part where 25 ≥ |(A∩B)∪C| = |A∩B| + |C| - |A∩B∩C| (shorter way to find the minimum). The result just ends up as |A∩B∩C| + 25 ≥ 20 which would result in a negative unless I can somehow divide both sides by -1 which would change the equality?