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Mine used the Best ACT Math Books Ever, books 1 and 2, to self prep for the math, but said that no matter how much they prepped, they still ran out of time on the math section, and reached problems that they hadn’t seen on the prep. Wound up with a 34 in math, 36 in everything else. Later on, after a few months of Calc BC class, said that had they had Calc before the test, they probably would have gotten a 36, because Calc could be used to solve problems faster. Odd - the test is not supposed to require Calc.
They didn’t use any other books, because they’d already absorbed a high level of English from reading and the level of spoken language in the home. They used the Official Guide to the ACT for some practice tests (you can get a slightly out of date one cheaply, since the only update they make is changing out one of the tests annually). You can also download legally released old ACTs from a thread on Reddit, usually, some with explanations.
For science, all they need to do is a few practice sections until they’ve seen all the ways in which data is presented. For English, do the sections to find what help student needs on points of grammar and learn those. If they’re having trouble with the reading passages, you do need a tutor, since this is a basic skill that is absorbed by having read widely throughout their school years. A good tutor one on one can help more with this than a prep class could.
If they’re starting way ahead of time, they could do one section a day. Mine did about 40-60 hours total, often did one math plus one “easy” section (Science, English, Reading) a day, but did go through the math sections in the book that were indicated by their first diagnostic test.
It is really very possible for a motivated student to self prep and do well on the ACT. However, most students just aren’t very motivated to take the time to do this, from what I’ve seen.