The number 20 squared is 20 times 20, or 400. To square a number means to multiply that number by itself. It is notated by writing the base number, which in this case is 20, with the number 2 in superscript after it.
Since 20 squared is 400, the square root of 400 is 20. The square root of a given number is the number that is multiplied by itself to yield that particular number. All squares are positive, but square roots are positive or negative. For example, the other square root of 400 is -20. Since multiplying two negatives makes a positive, -20 times -20 also equals 400.