Squaring numbers in the 20s
 Square the last digit (keep the carry) _ _ X
 Multiply the last digit by 4, add the carry _ X _
 The first digit will be 4 plus the carry: X _ _
Example:
If the number to be squared is 24:
 Square the last digit (keep the carry):
4 × 4 = 16 (keep 1) _ _ 6
 Multiply the last digit by 4, add the carry:
4 × 4 = 16, 16 + 1 = 17 _ 7 _
 The first digit will be 4 plus the carry:
4 (+ carry): 4 + 1 = 5 5 _ _
 So 24 × 24 = 576.
See the pattern?
If the number to be squared is 26:
 Square the last digit (keep the carry):
6 × 6 = 36 (keep 3) _ _ 6
 Multiply the last digit by 4, add the carry:
4 × 6 = 24, 24 + 3 = 27 (keep 2) _ 7 _
 The first digit will be 4 plus the carry:
4 (+ carry): 4 + 2 = 6 6 _ _.
 So 26 × 26 = 676.
