Edit: it appears that this is not exclusive to ADHD.
Posting this meme stemmed from my own efforts to explain my thought process when doing math and how it is similar to other people with ADHD doing math, while being different from every neurotypical person I'd talked to on the same subject.
While I didn't make the meme itself, instead finding it in my saves and wanting to share, I did accidentally spread misinformation that I had only backed up with personal anecdotal evidence.
I'll leave this up just so people can see the explanation below but this appears to not be ADHD related and just due to different people doing math in their heads differently...
Another neat trick: X% of Y is equal to Y% of X. That is, in your example, 27% of 65 == 65% of 27. So check and see which combination might provide fewer steps/messy numbers.
13.5 (50% of 27) + 2.7 (10% of 27) + 1.35 (5% of 27) = 17.55
Ahh, that's a really good point! I forget about the "X% of Y = Y% of X"
Honestly, I normally just use a calculator quick (move the decimal twice, multiply and all that jazz) for weird percentages or I want a precise answer.
But I like knowing different ways of thinking about it because it can become easier than using a calculator (with practice). And it's fun, cause I'm a bit of a math nerd