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This problem requires some simple knowledge of fractions and multiples and demands some strategic thinking. It may offer a good opportunity to compare methods between students - there isn't just one route to the solution. Note that there is no need to use algebra in this problem.
This printable worksheet may be useful: Ben's Game.
Choose three students to act out the scenario with a (real or imaginary) pot of $40$ counters, as described under support below.
Ask students to work in pairs or small groups to try and find the answer. If any groups are successful too quickly (!) ask them to change the total number of counters, or one or more of the fractions, and to adapt their strategies to the new situations.
As a group discuss the methods used.
What worked? What didn't work?
If faced with a similar problem in future, which methods would the class use?
Here are some possibilities: