Most people don't understand how percentages work.
Ask people these questions:
1) I'm going to sell you a TV. It's priced at $300. I'm going to give you a 10% discount. What will it cost after the discount?
2) I'm selling you a washing machine. You're paying me $500. It's already had a 10% discount. How much did it cost before the discount?
Most people can get an answer for (1), but they do it by mashing buttons on their smartphone calculator app. They don't know the method; they would not be able to talk through the method on paper. Most people can't give you an answer for (2) even after some time mashing buttons on their calculator.
People can't spot misuse of percentages (eg, the "politician's bridge" (also called the Mexican Bridge from How to Lie With Statistics))
This isn't just the general population. We find some scary lack of understanding of percentages in doctors.
https://en.wikipedia.org/wiki/Confusion_of_the_inverse
> In one study, physicians were asked to give the chances of malignancy with a 1% prior probability of occurring. A test can detect 80% of malignancies and has a 10% false positive rate. What is the probability of malignancy given a positive test result?[3] Approximately 95 out of 100 physicians responded the probability of malignancy would be about 75%, apparently because the physicians believed that the chances of malignancy given a positive test result were approximately the same as the chances of a positive test result given malignancy.[4]
> The correct probability of malignancy given a positive test result as stated above is 7.5%
Gerd Gigerenzer's book Reckoning with Risk has many examples of this.