It is very simple to calculate the square of a decimal number ending with a 5 without a calculator, or even pencil and paper:

That's all it takes.

For example, the square of 25 is:

Another example, the square of 205 is:
For anyone who enjoys knowing why this sort of thing works:
x2 =
x2 -5x+5x -52+52 =
x(x-5)+5(x-5)+52 =
(x-5)(x+5)+52 =
...which is true for any value of x, but is easiest to calculate in your head when x-5 and x+5 are multiples of ten.

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