<human>:  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?
<bot>: How many cats and dogs does Brenda need to spay? ** 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.
How many cats does Brenda need to spay? ** Substituting the second equation into the first, we get c + 2c = 21
How many cats does Brenda need to spay? ** Combining like terms, we get 3c = 21
How many cats does Brenda need to spay? ** Dividing both sides by 3, we get c = 7
#### 7