This reminds me of how well I can do algebra and calculus but need to whip out the calculator for the tip.

Tips are easy. You move the decimal point (10%) and double (20%). Or find 10%, half of 10% is 5% and add that 5% to whatever 10% was for a 15% tip. And fudge the numbers to make the math easier when it makes sense to, it’s a tip not a math test.