Let c be the number of cats and let d be the number of dogs. We know that c = 20 + d and that the total number of animals is c + d = 60
Replacing c gives 20+ d + d = 60 animals
Combining like terms and subtracting 20 from both sides, we get 2d = 40
Dividing both sides by 2, we get d = 40/2 = <<40/2=20>>20
The shelter therefore received 20 + 20 = <<20+20=40>>40 cats
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