Rachel drinks 2 glasses of water on Sunday and 4 glasses of water on Monday. Then she drinks 3 glasses of water every day for the next 4 days. One glass of water is 10 ounces of water. If Rachel wants to drink a total of 220 ounces of water in the week (Sunday to Saturday), how many glasses of water should she drink on Saturday?
Please reason step by step, and put your final answer within \boxed{}.
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