Study guides › Level 6 · Master

Chance (probability) from a bag

Level 6 · Master · for questions like “A bag holds 3 red, 5 blue and 2 green marbles. What is the chance of blue?”

💡 The idea

When every marble is equally likely:

chance = the number of the ones you want ÷ the number in all

🪜 The steps

1. Count the ones you want.
2. Count everything in the bag.
3. Write it as a fraction and simplify.

✏️ Worked examples

1. 3 red, 5 blue, 2 green. Chance of blue? 5 out of 10 = 1/2.

2. Same bag. Chance of not red? 7 out of 10 = 7/10.

⚠️ Watch out

• The bottom is everything in the bag, not just the other colors.
• Chance is never more than 1. If you get 5/3, it's upside down.

🎯 Try these

1. 4 red and 6 blue. Chance of red?
2. 2 green, 3 yellow and 7 purple. Chance of yellow?
3. 5 red and 15 blue. Chance of not red?

Answers: 1) 2/5 2) 1/4 3) 3/4

This guide is also in the 10MathQ app: tap “📖 Study guide” after a question.