This isn’t a riddle but a neat math trick my daughter figured out several years ago. If you take 2 numbers, x and x+2, their product will be one less than the square of their average. For example, 6×8=48, which is 1 less than the square of 7, or 49. This makes it easy to figure out a product like 19×21, since it’s just 20^2-1, or 400-1=399. If all of you already knew this, I apologize, but I thought it was neat (you can use polynomials to prove why this works).
A variation on this makes it easier to figure out the square of a number like 4.5. The answer is (4×5)+.25 or 20.25.
If anyone is interested I can post the proof. Or I can just leave it as an exercise for the reader (I always felt like this was a copout when I saw it in a textbook but it’s fun to write).