I am completely math illiterate. It takes me forever just to figure the tip on my bill.
One of my daughters is mathematically challenged as well. I taught her this trick for tipping.
Look at the total of he bill (78.53 for example).
Drop the rightmost digit, and adjust the sign in your head (7.85).
Round up to the next dollar, it never hurts to tip a little over (8.00)
If giving 15%, half that number and add it back ( 8.00 / 2 = 4.00. 8.00 + 4.00 = 12)
20% you double it and add it back. ( 8.00 * 2 = 16.00 )
She doesn't even need the tip calculator on her phone for tips now.
I figure out what 10 percent is first because that's easy. If the bill is 70 bucks, it's 7 dollars. Then I double it. So I would give 14 about.
Usually I just guess though. :P