01 / 12 · Count the chances
One out of four
Turn a count into a probability, without treating it as a promise.
Go to practice ↓A question to keep in mind
Four cards, one star. What is your chance of drawing the star?
Suppose every card is equally likely to be drawn. One of the four cards has a star, so the chance is 1 out of 4: 1/4, or 25%.
The shortcut is: count the outcomes you want, then divide by all equally likely outcomes. A 25% chance does not promise one star in the next four draws.
Work through an example
- Count all cards: 4.
- Count star cards: 1.
- Divide: 1 / 4 = 0.25 = 25%.
Your turn
0 / 3Draw once from these four equally likely cards. What is the chance of a star?
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25%
The number of labels is not the number of equally likely outcomes. Count the actual cards.
A box has 10 equally likely tickets: 3 blue and 7 white. Give each chance as a percentage.
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- Blue ticket: 30 %
- White ticket: 70 %
3 / 10 is 30%; 7 / 10 is 70%. The two possibilities cover every ticket, so they add to 100%.
You return the card and mix before each draw. With a 25% star chance, what can you say about the next four draws?
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There could be zero, one, or more stars.
25% describes the chance of each draw under these conditions, not a guaranteed count in four draws.
Bring it back to your own work
Make your own box with 10 tickets in two colors. Write the chance of each color and what assumption makes your calculation work.