10% of $36.24 is $3.60 (ignore the pennies)
Half of $3.60 is $1.80 (you can divide by 2, can't you?)
$3.60 + $1.80 = $5.40
$36.24 + $5.40 = $41.64, which is pretty darn close to the calculator-derived total of $41.68.
And no, I've never been accused of being a boring dinner companion.
What scares me is a restaurant full of the sort of people who worry about calculating tips to the third decimal...
Woops! You beat me to it. Actually, I dont know what it is about numbers that has so many people flustered... Must be the left wing whole language sociocrat neo-Marxists running the educational sytem in this country !