Square of a number ending with a 5 (thing)

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

• Chop off the 5;
• Multiply the rest (x) by itself plus 1 (x*(x+1));
• Append 25.

That's all it takes.

For example, the square of 25 is:

• Chop off 5, you get 2;
• Multiply 2 by 2 + 1, i.e., 2 * 3 = 6;
• Append 25, hence 25*25 = 625.

• Chop off the 5, you get 20;
• Multiply that by 20 + 1 = 21, i.e., 20 * 21 = 420;
• Append 25, hence 205*205 = 42025.