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. Since we normally count in tens, almost all the numbers that we come across are decimal numbers.
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The set of real numbers.
The number 1.68 belongs to the subsets of real numbers known as rational numbers and decimal numbers. As a rational number, 1.68 can be expressed as the ratio of two integers (84/50). It is also a decimal number, specifically a terminating decimal, where the digits after the decimal point eventually end.
Yes, it is. Moreover, it is also a rational number. 1.25 * 4 = 5, so 1.25 = 5/4. All rational numbers are real numbers, so 1.25 is real. Any number you can think of, using decimal notation is real. Real numbers are allowed to have an infinity of digits (behind the decimal point).
Real numbers are all numbers which do not contain "i", when "i" represents the square root of -1. All numbers which do contain "i" are "imaginary numbers" and are not real numbers. This means that all numbers you'd ordinarily use are real numbers - all the counting numbers (integers) and all decimals are real numbers. So in answer to your question, all the real numbers that are not whole numbers are all the decimal numbers - including irrational decimals such as pi.
The commutative property of addition applies to all real and complex numbers. It has nothing whatsoever to do with the form in which the number is represented: decimal, binary, etc.