The conversation between human and AI assistant.
[|Human|] Find the missing number in the following pattern: 12, 15, 18, 21, 24.
[|AI|] There is no missing number in the given pattern: 12, 15, 18, 21, 24. These numbers increase by increments of 3.
[|Human|] 
Find the missing number in the following pattern: 12, 15, 18, 21, 24.