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light one fuse from both ends. That takes 30 seconds to burn the fuse.
Light the second fuse at exactly the same time you light both ends of the first, but light one end only. That burns 30 seconds worth of the second fuse. Exactly at the moment when the first fuse has totally burned, light the second end of the second fuse. That takes 15 seconds to burn the second half of the second fuse. 30+15=45 seconds.
Light the second fuse at exactly the same time you light both ends of the first, but light one end only. That burns 30 seconds worth of the second fuse. Exactly at the moment when the first fuse has totally burned, light the second end of the second fuse. That takes 15 seconds to burn the second half of the second fuse. 30+15=45 seconds.