If one choice can happen in m ways and the next in n ways, and both happen, multiply. If you are choosing one or the other, add. That is the counting principle. Probability built on those counts is in probability problems.
Multiply the stages
A quiz has 4 reading passages and 5 math sets, and you do one of each. That is 4 × 5 = 20 pairings. Three shirts and 2 pairs of shoes, one of each, is 6 outfits. Order matters in these when the stages are different jobs. When order is the whole question, switch to permutations and combinations.
Add only when the paths are separate
A student takes the bus or walks, 2 bus routes and 1 walking route: 2 + 1 = 3 ways, because she does not do both. Multiplying there double-counts. Statistics items that start from a count are in statistics and probability. Overlaps, where adding would double-count, are in Venn diagrams.
The bottom line
And means multiply. Or, when the options cannot happen together, means add.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
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