If calculating it by division, it is when the remainder is 0. Otherwise it is not possible to be sure.
27.9 IS a decimal number. You can tell it is a decimal number because it has a decimal point (.).
you can tell By looking at the first decimal.
When a decimal can't be expressed as a fraction then it is an irrational number.
7
You can if the last digit of the hundredth is zero.
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The same way you can tell with integers.
4.4
4.0
If it is a terminating or recurring decimal then it is not irrational. If it is an infinite, non-recurring decimal, it is irrational.
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