The expression 'r - 27' is not equivalent to '27 - r' because it represents a different mathematical operation, while the other expressions are just rearrangements or involve neutral operations like adding zero or multiplying by one.
The expression that is NOT equivalent to 27 - r is r - 27. The other expressions given, (27 - r) + 0, (27 - r) × 1, and -r + 27, are all equivalent to 27 - r. The reason is that adding zero to a number does not change its value, multiplying a number by one leaves it unchanged, and the subtraction in the form of -r + 27 is just a rearrangement of 27 - r.