Coolest math trick I ever learned was multiplication by 11.
Any number X 11 = the left digit, the sum of the left and right digits, the right digit
Example: 23 X 11 = 2 then 5 (2+3) then 3 = 253
Example: 48 X 11 = 4 (temporary) then 12 (temporary) then 8. Take the 1 from the 12 and add it to the 4 to make 5, keep the 2, then 8 = 528
Example: 534 X 11 = 5874
Example: 8639 X 11 = 95029
It gets trickier the longer the number you’re multiplying, so for those it’s easier to start from the right and move left, but it works and people think you’re a math genius when you’re not really.
Cool, and past me.
And, sorry for the slide. I’m out.