Every event has a complement — the chance it does NOT happen — and the two always add to one whole. These free printable worksheets practice finding P(not) from P(event), with an answer key.
Kid stuck on one? You don’t just get the sheet.
The answer key prints on a separate page.
The rule is P(not) = 1 − P(event), and the reason it works is that something either happens or it does not, with no third option: the two probabilities cover every outcome, so together they make one. If the chance of drawing red is 1/4, the chance of not-red is 3/4, and the two summing to 4/4 is the check built into the idea.
There are two honest routes and comparing them is the lesson. A child can count everything that is not the event and write that over the total, or take the event's probability and subtract it from one — both give the same fraction. Working it both ways, and simplifying the result, is what turns the rule from something memorized into something seen.
One question from this sheet, worked through a step at a time.
5 red, 7 blue — P(not red)