Unit 12 · Multi-Step Conditional Reasoning
Lily & Tom — How Many Cards to Be Certain?
Lesson 15 of 16
About this lesson
Darren has a 15-card deck with 5 red, 5 blue, and 5 yellow cards numbered 1–5. Lily says drawing 11 cards guarantees all three colours; Tom says drawing 7 cards guarantees three different numbers. Are they right? This question introduces one of the most powerful reasoning patterns in OC TS: worst-case thinking. Instead of asking "what might happen?", you ask "what is the worst that can happen, and when does even that worst case guarantee the result?"
Chapters
Question
| Darren has a deck of fifteen special playing cards:
- Five red cards, numbered 1–5
- Five blue cards, numbered 1–5
- Five yellow cards, numbered 1–5
| He shuffles the deck randomly, and then draws a number of cards from the top. |
|---|
Lily: "To be certain of having all three colours, he has to draw eleven cards."
Tom: "To be certain of having three different numbers, he needs to draw seven cards."
If the information in the box is true, whose reasoning is correct?
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