I wouldn't label it ACT Math "Have to Know", but rather ACT Math "Good to Know," which simply means that this math idea is not something tested on every single ACT Math test, but it is certainly required on most tests. If you'd rather, I break it all down on YouTube here with an example.
However, the idea is pretty simple, and once you can recognize it, getting these questions correct regularly becomes much easier.
The Fundamental Counting Principle is a way to determine how many possible ways there are to do a thing. It states that if there are p ways to do 1 thing and q ways to do another thing, then there are p x q ways to do both.
For example, here is a fairly common way this question is asked (though I break down some different, trickier ways in the YouTube video linked above).
Imagine there is an ice cream shop that offers 4 types of cones, 11 types of ice cream, and 15 different toppings. If every order consists of 1 type of cone, 1 type of ice cream, and 1 topping, then how many ways are there to order the ice cream?
When students do the math on this incorrectly, they usually make the mistake of adding these numbers together (they say, 4 + 11 + 15 = 30!). Then, they select this option and get it correct.
However, it is not necessary to add them, but rather to multiply them:
4 x 11 x 15 = 660!
That's it! Give the parameters, there are 660 different ways to order an ice cream at this particular ice cream store!
If you want more: more examples of this idea, more lessons on ACT Math that is "Good to Know" vs "Have to Know", and 3 full-length practice tests, check out The ACT Math System book!
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If you want some free ACT prep cheat sheets that lay it all out in a few pages, then click here!
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