What is the common difference between the elements of the arithmetic sequence below? –18, –22.5, –27, –31.5, –36

-22.5-(-18)=-4.5
-27-(-22.5)=-4.5
ans -4.5

The first one, i believe, would be correct. all the others have one thing in common, and that's that they have repeating numbers. in the second one, x and y have repeating 6's and 3's, and in the third, x and y have repeating 6's as well. in the last one, there are repeating 3's. the first one has no repeating numbers.