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?
assistant
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