<human>:  Lee rears only sheep and geese on his farm. If the total number of animal legs is 70, and the total number of animal heads is 20, how many sheep live on Lee's farm?
<bot>: Define a variable ** Let g be the number of geese and let s be the number of sheep. The farm has g + s = 20 animals.
How many sheep does Lee have? ** Therefore Lee has s = 20 - g sheep.
How many legs do the animals have? ** The animals have 2g + 4s = 70 legs.
Write an equation ** Using the value of s above, 2g + 4(20-g) = 70.
Simplify ** 2g + 80 - 4g = 70.
Simplify ** 2g = 10.
How many geese are there? ** There are therefore 10/2 = <<10/2=5>>5 geese.
How many sheep are left? ** There are 20 - 5 = <<20-5=15>>15 sheep remaining on the farm.
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