The only rule is 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.
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If there are any numbers which are integers and so do not have a decimal point, then append one at the extreme right. Then arrange all the numbers in a column, with their decimal points aligned. Ad up the numbers ignoring the decimal points entirely. In the answer insert a decimal point under the column of decimal points.
Line up the decimal points in a vertical column. Then add the numbers while ignoring the decimal point. Finally, put a decimal point in the answer in the same column as for the summands.
None of the following rules are applicable.
Whole numbers are a proper subset of decimal numbers. All whole numbers are decimal numbers but not all decimal numbers are whole numbers.
Any amount of numbers can be in a decimal.