鸡兔同笼：笼里有8个头,26只脚.鸡和兔各有几只?
设有兔x只,则鸡有（8-x）只.
4x+2(8-x)=26 8-5=3
4x+2*8-2x=26 有兔5只3只鸡
2x =10
x =5