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.
If they get stuck
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 a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Complementary events, step by step