Let c be the number of cats Brenda needs to spay and d be the number of dogs.

We know that c + d = 21 and d = 2c.

Substituting the second equation into the first, we get c + 2c = 21

Combining like terms, we get 3c = 21

Dividing both sides by 3, we get c = 7

# Answer

7
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Brenda is a vet who needs to spay some cats and twice as many dogs. If she needs to spay 21 animals total today, how many cats does she need to spay?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
7
Brenda is a vet who needs to spay some cats and twice as many dogs. If she needs to spay 21 animals total today, how many cats does she need to spay?
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