6. A young sprinter ran 90 meters with the wind in 10 seconds. Under the same wind speed, he ran 70 meters against the wind in 10 seconds. Question: In the absence of wind, how many seconds would it take him to run 100 meters?
6. It takes 12.5 seconds to run 100 meters without wind
6. 【Solution】Speed with the wind $=90 \div 10=9$ (m/s), speed against the wind $=70 \div 10=7$ (m/s), speed without wind $=(9+7) \times \frac{1}{2}=8$ (m/s), it takes $100 \div 8=12.5$ (seconds) to run 100 meters without wind. Answer: It takes 12.5 seconds to run 100 meters without wind.