SQUARING ANY TWO-DIGIT NUMBER ENDING IN 5
Squaring a two-digit number ending in 5 is a special case of the short cut for multiplying any two-digit numbers ending in 5. In this particular case, the tens digits are equal.
Rule: Multiply one more than the tens digit by the original tens digit and affix 25 to the result.
For example, we shall square 45. First, add 1 to the tens digit.
4 + 1 = 5
Next, multiply by the original tens digit.
4 x 5 = 20
To this affix 25.
2,025
and we have the answer.
45 x 45 = 2,025
Remember to merely attach the 25 to the product: do not add it to the product.
From this rule we see that the square of any two-digit number ending in 5 always has 5 as its units digit and 2 as its tens digit.