Question 852422
Is there more to this problem?  Do adults and children eat the same amount?  How large a slice?
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If this is a real-world question, {{{1/2}}}sheet will make certain everyone has an ample amount.  You can get away with {{{1/4}}}sheet, but the slices will be small.
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http://www.hurryupcakes.com/general-cake-information/how-many-people-does-a-sheet-cake-feed