I have a problem for which I know the answer, but have no idea why it works.
Of 40 people, 30 have a condition. 15 have condition X and 25 have condition Y. How many people have both X and Y?
I know the answer is 10 (thanks to the answers sheet) but why? When I simulate the question by giving random people X's and Y's, and count the people with both, I get answers ranging from 8 to 13. Is my simulation off? Is my thinking wrong?
Why would it be 10, all the time?
Only theory is 15+25=40, 40-30=10, but this doesn't seem logical. (25-15=10, but that's just coincidence right?)