Sarah is planning a road trip and has a total of 15 gallons of gasoline in her car. She drives to a beach that is 10 miles away, spends some time there, and then drives back home. After that, she heads to a mountain which is 12 miles away, but halfway through the journey, she realizes she forgot her hiking gear at home, so she drives back home to get it and then drives to the mountain again. If she now has exactly 5 gallons of gasoline left, at what rate in miles per gallon is her car consuming gasoline? Let's think step by step and output the final answer within \boxed{}.
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