



 
Peter, I can help with b). If half the girls sit with boys then the number of girls must be even. Now consider the girls who are not sitting with boys. Each of them sits with another girl so this number of girls is even also. Thus the number of girls is divisible by 4. Now suppose they are rearranged and half the boys sit with girls. What can you say about the number of boys? I hope this helps,  


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