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Mathematics problems (fractional equation) Set up fractional equation to solve the problem: the design speed of Fuzhou Third Ring Road is 80 per hour. It is known that the distance between a and B is 20 km. Xiao Zhang walks along the Third Ring Road at an average speed of 80 per hour, which is 15 minutes shorter than the original speed. Find out Xiao Zhang's original speed. (the distance between a and B is 20 km when taking the original road and the third ring road.)
Let's set the original speed x km / h
It takes 20 / X hours for Xiao Zhang to complete 20 kilometers at the original speed,
It takes Xiao Zhang 20 / 80 hours to walk 20 km in 80 km / h
15 minutes (1 / 4 hour) shorter than before
20/x-20/80=1/4
x=40
A: Xiao Zhang's original speed was 40 km / h
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