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How many different groups of 6 children can be chosen from a class of 18 children if the class contains one set of twins who must not be seperated?

davidchoo12
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Consider two cases: the twins are in the group of $6$, or not.

If the twins are in the group of $6$, then it remains to choose $4$ more children from the remaining $16$.

If the twins are not in the group of $6$, then you need to choose $6$ students from $16$.

angryavian
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