For the most part, yes. However, it's important to remember that there are usually some relatively easy questions towards the end of a section (18+) and a few relatively difficult questions in the middle (12+). Those numbers are just estimates - mostly what I have noticed.
Yeah, difficulty seems to peak around about number 20-22. The trend from easier to harder is only a trend though. There is nothing stopping them from throwing a curve breaker in on number 3 or anywhere else. There's no real way to say that number 18 will be harder than number 4, or anything else that specific. But the average difficulty of numbers 15-20 will be higher than the average difficulty of numbers 1-5.
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Those numbers are just estimates - mostly what I have noticed.