You can get a few of them just by assuming that one of the choices is correct and figuring the last digit.
41 x 21 = (40 + 1) x (20 + 1) = 800+ 40 + 20 +1.
Only one of the choices ends in 1. Don’t bother doing the sum. And, no, I didn’t need paper and pencil.
I got most that way... not a very good test for figgering
Gee, your way of determining 41 x 21 looks more complicated than mine.
41 x 21 = 41 + 820 ... or 21 + 840
I’ve been doing math in my head since grade school.