Please design a math exam question: by designing a hierarchical " problem string ", so that students can expose the thinking process in the answer to the question.

2. Master Zhang driving a heavy car from the county to the provincial city delivery, arrived in the provincial city immediately unloading and then return along the road, he drove the car to go to 64 kilometers per hour, return to 56 kilometers per hour, round trip **** 12 hours (unloading time in the provincial city), Master Zhang in the provincial city and the county to go to the 64 km per hour, return to 56 km per hour, a round trip **** 12 hours (in the provincial city unloaded time), Master Zhang in the provincial city and the county to go to the provincial city and the county to return to the provincial city, he was not a good choice. Neglected), Master Zhang in the province and the county between a round trip *** line a few kilometers?

3. (400/x - 5) (x+4) - 400 = 400+9x

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