Squaring numbers in the 80s
 Square the last digit (keep the carry) _ _ X
 Multiply the last digit by 16, add the carry _ X _
 The first digits will be 64 plus the carry: X X _ _
Example:
If the number to be squared is 83:
 Square the last digit:
3 × 3 = 9 _ _ _ 9
 Multiply the last digit by 16:
16 × 3 = 30 + 18 = 48 _ _ 8 _
 The first digits will be 64 plus the carry:
64 (+ carry): 64 + 4 = 68 6 8 _ _
 So 83 × 83 = 6889.
See the pattern?
If the number to be squared is 86:
 Square the last digit (keep the carry):
6 × 6 = 36 (keep 3) _ _ _ 6
 Multiply the last digit by 16, add the carry:
16 × 6 = 60 + 36 = 96 96 + 3 = 99 (keep 9) _ _ 9 _
 The first digits will be 64 plus the carry:
64 (+ carry): 64 + 9 = 73 7 3 _ _
 So 86 × 86 = 7396.
