Earlier quoted context omitted.
> people with autism are more likely than the general population to have ADHD, but people with ADHD are not more likely than the general population to have autism I'm pretty sure that's not mathematically possible. If W people have both, X people have only autism, Y people have only ADHD, and Z people have neither, can you provide values for W, X, Y, and Z that would make that claim true?
W (9), X (1), Y (9), Z (1) General chance of being in either group: 50% (10 people with autism, 10 with ADHD, 20 total). Chance of having autism if you have ADHD: 50% (9 with autism, 9 without, 10 total). Chance of having ADHD if you have autism: 90% (9 with ADHD, 9 without, 10 total).
This is wrong. With your numbers ("W (9), X (1), Y (9), Z (1)"), 18 people would have ADHD, not 10.