2. 外语
  3. In a driving competition, Jeff and Dennis drove the same course at average speeds of 51 miles per hour and 54 miles per hour, re

In a driving competition, Jeff and Dennis drove the same course at average speeds of 51 miles per hour and 54 miles per hour, re

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问题 In a driving competition, Jeff and Dennis drove the same course at average speeds of 51 miles per hour and 54 miles per hour, respectively. If it took Jeff 40 minutes to drive the course, how long did it take Dennis?
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答案37.8 minutes
解析 Let x be the time, in minutes, that it took Dennis to drive the course. The distance d, in miles, is equal to the product of the rate r, in miles per hour, and the time t, in hours; that is,
d=rt
Note that since the rates are given in miles per hour, it is necessary to express the times in hours; for example, 40 minutes equalsof an hour. Thus, the distance traveled by Jeff is the product of his speed and his time,(51)()miles, and the distance traveled by Dennis is similarly represented by(54)()miles. Since the distances are equal,

Thus, it took Dennis approximately 37.8 minutes to drive the course.
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