Sam sells bread. He has a target of selling 120 crates of bread in a week. One week he was closed on Monday and Friday. Over the weekend he sold 20 crates. On Tuesday he sold 15 crates, on Wednesday 12 crates, and Thursday 18 crates. By how many crates was Sam off from his target for the week?
Please reason step by step, and put your final answer within \boxed{}.
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