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In a farm, there is an equal number of cows, sheep, and chickens. These animals have $250$ legs in total. How many sheep are there? 
Overseas Competition->Knowledge Point->Word Problem Modules->Chicken-Rabbit Problems
Group $1$ of each kind of animals together, and there are $4+4+2=10$ legs in each group. There are $250\div10=25$ groups, so there are $25$ sheep. 