Mingming, Panpan, and Nana start from school to go to the park at the same time. Mingming's speed is 20 meters per minute, and Panpan's speed is 16 meters per minute. At the same time, Dongdong walks towards them from the park. Dongdong meets Mingming after 6 minutes, meets Panpan after 7 minutes, and meets Nana after 8 minutes. What is Nana's speed?

### Let's think step by step and output the final answer within \boxed{}.
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