Mary is chopping up some old furniture to make firewood. Chairs make 6 sticks of wood, tables make 9 sticks of wood, and stools make 2 sticks of wood. Mary needs to burn 5 sticks of wood per hour to stay warm. If Mary chops up 18 chairs, 6 tables and 4 stools, how many hours can she keep warm?
Please reason step by step, and put your final answer within \boxed{}.
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