In a two-circle Venn diagram, add the two groups, then subtract the overlap so you do not count those people twice. The picture is inclusion. Probability from the same counts is in probability problems.
The overlap
18 students take math, 12 take science, and 5 take both. The number in at least one is 18 + 12 − 5 = 25. The 5 live in the lens, not in a third group you add on. Only math is 18 − 5 = 13. Only science is 12 − 5 = 7. Neither, if the whole school is 30, is 30 − 25 = 5.
“And” is the overlap. “Or” is the union, which is the sum minus the overlap. Multiplying independent stages is a different job, in the counting principle.
Three circles
A third circle adds regions that sit in two overlaps and one triple overlap. Fill the very center first, then the pairwise lenses, then the only-one regions. Statistics wording around these counts is in statistics and probability. Unordered selections that are not about overlap are in permutations and combinations.
The bottom line
Draw it, fill the overlap first, and subtract that overlap when you want the total in either group.
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.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.