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

### Instruction:
Tom hasn't been sleeping well lately. He figures he has been getting about 5 hours of sleep each weeknight and 6 hours each night on the weekend. If Tom would ideally like to get 8 hours of sleep each night on both weeknights and weekends, how many hours of sleep is Tom behind on from the last week?

### Response:
Tom got 5 hours of sleep for 5 nights, and 6 hours of sleep for 2 nights.
That's 5 * 5 + 6 * 2 = <<5*5+6*2=35>>35 hours of sleep.
He should have gotten 8 hours of sleep for 7 nights.
That's 8 * 7 = <<8*7=56>>56 hours of sleep.
He is behind by 56 - 35 = <<56-35=21>>21 hours of sleep.
#### 21

