All together, step by step
A set has 12 cards: 4 red, 4 blue, 4 green. Each colour has the numbers 1, 2, 3, 4. You pick one card without looking. What is P(not red)?
Step 1 of 4
- Count all: 12 cards. Not 3 colours!
- Count wanted for red: 4 cards. P(red) = 4/12 = 1/3.
- Use the rule: P(not red) = 1 − 1/3 = 2/3.
- Check: blue + green = 8 cards. 8/12 = 2/3. It matches.
Not red is 8 cards of 12. The chance is 2/3.
The five steps
- Count all the outcomes.
- Check they are equally likely. Same-size cards, well shuffled: yes.
- Count the wanted outcomes.
- Use the rule the question needs: ÷, "not", or "or".
- Check another way.
One more
Same cards. What is P(a 1 or a 4)?
- Cards with 1: 3. Cards with 4: 3.
- No card has both, because each card shows one number. So add: 3 + 3 = 6.
- P = 6/12 = 1/2.
Watch out!
Counting 2 colours out of 3 also gives 2/3. That number is right here, but the method is wrong. Try 5 red, 4 blue and 3 green cards. Colours still say 2/3. The cards say 7/12. Count cards, not colours.
Now try it. Tap the cards that fit each question.
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