You write it as 12, exactly as in the question.
A decimal number is simply a way of representing a number in such a way that the place value of each digit is ten times that of the digit to its right. A decimal representation does not require a decimal point. Adding zeros after the decimal point is wrong because they imply a degree of accuracy (significant figures) for which there is no justification.
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12 and one half as a decimal is 12.5
If you mean 11 and 12/100 then as a decimal it is 11.12
12 and eight tenths as a decimal is 12.8
12/20 = 6/10 so the decimal is 0.6
12/10 = 1.2