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The sets of 10 marbles are chosen randomly, so could contain:
4 red, 3 green, 2 blue and 1 yellow or
4 red, 3 green and 3 yellow or
7 red and 3 blue or...
A run of 10 viewings may produce an outcome that is not representative of the actual contents of the bag.
In the Bag
Age 11 to 14
Challenge Level 
The sets of 10 marbles are chosen randomly, so could contain:
4 red, 3 green, 2 blue and 1 yellow or
4 red, 3 green and 3 yellow or
7 red and 3 blue or...
A run of 10 viewings may produce an outcome that is not representative of the actual contents of the bag.
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