"until probably 100 or 144" - ugh. That's 10 and 12. I hope he 's wrong about that and schools aren't that bad where he lives. In Germany we've had to memorize squares for numbers 1 to 20 and 25, which gives you the roots of 1, 4, ..., 100, 121, ..., 400, 625. For every other number beetween these, you can estimate the root with two decimal places pretty well.