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Answer: 29 rungs

The firefighter is standing on the middle rung. He goes up 6 rungs so he is now at M+6, he now goes down 10 rungs so he is now at M-4 he goes up 18 rungs so he is now at M+14=T. 14 rungs from the top, 14 rungs from the bottom and the middle rung. 14+14+1=29!

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