Christmas puzzle

The Dutch equivalent of the CIA & NSA is called the AIVD. One of their departments has been compiling a set of Christmas puzzles for decades, and since a few years these puzzles are published on the internet. You can download the 2016 version.

Though most puzzles are language dependent (in Dutch), there are some which at least on the surface do not seem to require knowledge of the Dutch language.
Here is a list of the exercises and the translation of the exercises/hints of the puzzles for which you probably don’t need to know dutch:
2. Elementary: Which one is out of order?
4. What is the next number in each of the two series?
10. Two persons on a ferry are comparing two rows. One counts differences, the other comparisons. They arrive at the following series. What are the next numbers?
23. Sequences. What are the next three items in the lists?
I warn you, they have the reputation to be pretty tough. 100 points can be earned each year. Every year, people crack all exercises, but in no year did one person all problems.

Thanks to our daughter Joella, who solved puzzle 1b, I can offer you the puzzle below. Which number should replace the question mark:

No, I don’t intend to publish the solutions. But I guess the solutions will be published here


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