The chance of one thing happening is how many ways it can, over how many ways anything can. These free printable worksheets describe a bag of colored counters and ask for the probability as a fraction in its simplest form.
If they get stuck
The answer key prints on a separate page.
Four blue counters in a bag of twelve gives 4/12, which is 1/3. Both steps matter and only the second one is arithmetic: the first is deciding what the denominator is. A child who counts the blue ones and the red ones and writes 4/8 has used the wrong whole, and it is the mistake that survives longest.
Some questions ask for the chance of something NOT happening, which is the same count read the other way. Eight of twelve are not blue, so the answer is 2/3 — and noticing that it is one minus the first answer is the beginning of a rule worth having.
One question from a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Probability, step by step