The event (given) The complement (find it) The whole bar = 1. Take away the event, what's left is its complement.
A spinner lands on brass with probability 3/8. What's the chance it does not land on brass?
Step 1 — Write the ruleP(not A) = 1 − P(A)
Step 2 — Put the number inP(not A) = 1 − 3/8
Step 3 — SubtractP(not A) = 5/8
The complement is whatever's left over from the whole. The whole is always 1, so P(not A) = 1 − P(A). Type your answer as a fraction (like 5/8) or a decimal (like 0.625).