If P(red) = 2/5, P(not red) = ?

**The reasoning** This is about **complementary probability** — one of the most fundamental rules in probability. The key principle: all possible outcomes must add up to 1 (or 100%). Think of it like this: every ball is either red or NOT red. There's no third option. So: P(red) + P(not red) = 1 Given P(red) = 2/5, we substitute: 2/5 + P(not red) = 1 P(not red) = 1 − 2/5 = 5/5 − 2/5 = **3/5** **Why the wrong options tempt you** **Option A (2/5):** You might think "not red" equals "red" — but that only works if outcomes are equally split 50-50, which isn't the case here. **Option C (1/5):** Maybe you subtracted incorrectly or confused this with something else entirely. **Option D (1):** This would mean nothing is red at all, contradicting the given information. **Quick takeaway** Whenever you see P(event), remember: P(event) + P(not event) = 1 always. Subtract from 1 to find the complement — you'll never miss this again! 🎯