I think I am missing something here. People are told the initial price and a few minutes later the sale price and are asked to _estimate_ the savings as a percentage of the original? Are we this horribly bad at basic fourth grade math?
You basically need to do a calculation like this in your head:
100*(3 - 2.33)/3 ≈ 22.33%
When I went out to dinner with my parents, none of the ten people over age 30 could calculate a 20% tip - much less the other way around.
Most people just do not do math outside the presence of a calculator, so those skills fade.
Perhaps I'm getting ahead of myself but it seems like a slippery slope to rely on a calculator for (subjectively) simple calculations. It leads to a "oh, someone else will figure this out for me" mentality.
Finding 10% of an amount is easy. All you have to do is move the decimal point over one position to the left. For example if you had an $80 dinner, then 10% of that is $8. Just double that amount to find the 20% tip, in this case $16.
I think that with a little thought most people should be able to figure out these tricks, but you are right, most people don't bother and their skills fade.