In order to find the common difference in an arithmetic sequence, we need to find the difference between any two consecutive terms. The sequence given is:
[tex]\[ -18, -22.5, -27, -31.5, -36 \][/tex]
1. Pick the first term and the second term in the sequence:
[tex]\[ -18 \][/tex] and [tex]\[ -22.5 \][/tex]
2. Subtract the first term from the second term to find the common difference:
[tex]\[ \text{Common difference} = -22.5 - (-18) \][/tex]
[tex]\[ = -22.5 + 18 \][/tex]
[tex]\[ = -4.5 \][/tex]
Thus, the common difference in the arithmetic sequence is:
[tex]\[ \boxed{-4.5} \][/tex]
