That's why it's an abuse of the "number close to 50" trick. It really is meant for single digit differences from 50. The only reason it works for 75 is because

That's why it's an abuse of the "number close to 50" trick. It really is meant for single digit differences from 50. The only reason it works for 75 is because hopefully 25^2 comes up often enough that people have it memorized. I know I memorized it long ago after seeing it a number of times: "twenty-five times twenty-five? six twenty-five" is easy for me to remember. If people don't have 25^2 memorized, the trick kind of falls down. Otherwise, you're right. You can break down the square of any number into (x+y)(x+y) = x^2 + 2xy + y^2. For 50, the "2x" term equals 100, and multiplying by 100 and adding to 2500 is dirt simple for anybody to do.