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Twelve food units shared among four of ten kids

Question

A babysitter looks after 1010 hungry children and has 1212 identical units of food. She decides that exactly 44 of the children will be fed and the other 66 get nothing. In how many ways can she hand out all 1212 units so that each of the chosen 44 children gets at least one unit?

Short answer

34,65034{,}650. Choose the four fed children in (104)=210\binom{10}{4}=210 ways. Give each of them one unit first, then share the remaining 88 units freely among the four, which by stars and bars can be done in (113)=165\binom{11}{3}=165 ways. In total 210⋅165=34,650210\cdot165=34{,}650.

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