Below is an instruction that describes a task. Write a response that appropriately completes the request in English. Please answer in English.

### Instruction:
Melody has three dogs. Each dog eats 1/2 of a pound of dog food twice a day. If Melody bought 30 pounds of dog food, how many pounds of dog food are left after a week?

### Response:
Each dog eats 1/2 x 2 = <<1/2*2=1>>1 pound of dog food each day.
So the three dogs eat a total of 1 x 3 = <<1*3=3>>3 pounds each day.
Therefore, the three dogs can eat 3 x 7 = 21 pounds in a week.
Thus, 30 - 21 = <<30-21=9>>9 pounds of food are left after a week.
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