In order to do math in your head, you have to have certain basics memorized, and you combine these to come to a solution. It’s pretty common to learn the perfec

In order to do math in your head, you have to have certain basics memorized, and you combine these to come to a solution. It’s pretty common to learn the perfect squares up to 12 or so 1x1 = 1 2x2 = 4 3x3 = 9 4x4 = 16 5x5 = 25 6x6 = 36 7x7 = 49 8x8 = 64 9x9 = 81 10x10 = 100 11x11 = 121 12x12 = 144 If you have that readily accessible in your head, you can simply do a linear scan until you find which two numbers the square is between. In practice you will probably skip a few or just recognize it as being close after some practice, but that’s the general idea. As an aside the perfect squares are just a portion of the single digit times table that you would want in your head to do multiplication or long division.