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?
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
#### 7
Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
You are solving math problems. Here are some examples of how to solve the task:

Sebastian bought art exhibit tickets for his parents and himself.  Tickets were $44 per person.  He was also charged an $18 service fee for the online transaction.  What is the total amount he paid for the tickets?Tickets for Sebastian and his parents cost $44 x 3 = $<<44*3=132>>132.
Tickets and service charge cost $132 + $18 = $<<132+18=150>>150.
#### 150[END]

Brooke is milking cows and then selling the milk at the market for $3 a gallon. Whatever milk doesn't sell, she turns into butter. One gallon of milk equals 2 sticks of butter. She then sells the butter for $1.5 a stick. She has 12 cows. Each cow produces 4 gallons of milk. She has 6 customers, each of whom wants 6 gallons of milk. How much money does she earn if she sells all her milk and butter?A gallon of milk makes $3 worth of butter because 2 x 1.5 = <<2*1.5=3>>3
A gallon of milk sells for the same amount as milk or butter because 3 = <<3=3>>3
She gets 48 gallons of milk because 4 x 12 = <<4*12=48>>48
She earns $144 selling the milk because 48 x $3 = <<48*3=144>>144
#### 144[END]

Now, solve this next question in the style of the above examples:

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?

### Response:
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
#### 7