Include–Exclude as Case Analysis

Count at least one by complement or add/subtract overlaps with a Venn mindset.

Try this in Python

universe = 100
bad = 33 + 20 - 6  # |A∪B| toy
print(universe - bad)

Key takeaways

• Draw two-set Venn before three-set formulas.
• Complement flips “none” vs “some”.
• Practice mod arithmetic if required.