Xena is trying to outrun a dragon to get to the safety of a cave it's too big to fit into. Xena has a 600 foot head start, but the dragon can burn her if it gets within 120 feet of her. If Xena runs 15 feet per second and the dragon flies 30 feet per second, how many seconds does Xena have to get to the cave?
Please reason step by step, and put your final answer within \boxed{}.
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