It could be 18
The problem here is there are only 3 terms to consider, which makes it difficult to determine if the sequence is geometric or arithmetic
If we consider the difference in the terms
30 - 24 = 6
24 - 20 = 4
The differences are not constant so we can only guess at what the next term might be
the difference in the next terms might be 2 , following the pattern subtract 2 from each difference
20 - ? = 2 gives ? = 18
