That's not a smart-aleck answer. It's the best I (or anybody) can do.
The only way to answer the question (or any question like it) is to ask the person who wrote the problem what the pattern is.
You can put any number you want in the blank and it will be a valid sequence.
Presumably there is a logical pattern that was used to create the problem. But there are probably other logical patterns in the given numbers which would produce a different missing number.
So unless a problem like this tells you something about what kind of pattern to look for, it is a waste of time.
