What will be the unit digit of the square of 639?

For calculating the unit digit of a number when squared, just square the unit digit of the number and obtain the value.
For example: unit digit of square of 32 = square of 2 = 4
Therefore, unit digit of square of 639 = Square of 9 = 9 × 9 = 81

So, 1 will be the unit digit of square of 639.