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The Lady or the Lions

The King showed the Princess a map of the maze and the Princess was allowed to decide which room she would wait in. She was not allowed to send a copy to her lover who would have to guess which path to follow. Which room should she wait in to give her lover the greatest chance of finding her?

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A Dicey Paradox

Four fair dice are marked differently on their six faces. Choose first ANY one of them. I can always choose another that will give me a better chance of winning. Investigate.

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Nines and Tens

Explain why it is that when you throw two dice you are more likely to get a score of 9 than of 10. What about the case of 3 dice? Is a score of 9 more likely then a score of 10 with 3 dice?

In a Box

Stage: 3 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Why do this problem?

This problem offers opportunties to consider different methods of listing systematically. It can be used to introduce or revisit sample space diagrams, and with some students, tree diagrams.

Possible approach

Play the game a few times for real.
"Is this a fair game? How can we be sure?"
Class work in pairs trying to decide and to develop an argument to justify their conjectures.
After about ten minutes, stop to discuss the merits of different arguments and representations. This may be an appropriate point to highlight the benefits of different systematic methods for listing all possibilities, using sample space diagrams and, if pupils have met them before, tree diagrams.

Finding a fair game can become a class activity:
Students help to create a class list of all distinct starting points for the game (for example, four ribbons can be either $1R$ and $3B$ or $2R$ and $2B$). These are written on the board for $3, 4, 5, \ldots$ribbons.
Distribute the task of checking which combinations are fair and record them on the board as pairs of pupils decide.
There are not many solutions that work and if pupils are to notice a pattern amongst the combinations that are fair they may need to consider up to a total of $16$ ribbons.

Spend some time conjecturing about more than $16$ ribbons and test.

Key questions

How can you decide if the game is fair?
How many goes do you think we need to be confident of the likelihood of winning?
Are there efficient systems for recording the different possible combinations?
Can you justify your conclusions?

Possible extension

Justifying the general rule

Possible support

When the work is being shared out amongst pupils in the class the smaller number of ribbons can be given to pupils who struggle most. They can also use physical objects, such as coloured counters, to check their findings.