<human>:  Farmer Brown has 20 animals on his farm, all either chickens or cows. They have a total of 70 legs, all together. How many of the animals are chickens?
<bot>: Define a variable ** Let C be the number of chickens.
How many cows are there? ** There are 20-C cows.
How many legs do the cows have? ** The cows have 4*(20-C) legs.
How many legs do the chickens have? ** The cows have 2C legs.
Write an equation using the total number of legs ** The total number of legs is 2C+4(20-C)=70.
Simplify ** 2C+80-4C=70
Combine like terms ** 2C=10
Divide by 2 ** C=<<5=5>>5
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