This topic is a travel topic. The trip problem is nothing more than the relationship between distance, speed and time. The simple method used in your analysis, the special value method. Suppose the distance is 65×60, then we know that it will take 65 hours to reach the destination. We know that the average speed is 65, so the average time is 60. When the time to reach the destination is 65 and the average time is 60, the return time is 55. Then the speed of the return trip is equal to the distance divided by the time = 65× 60/55 ≈ 7 1. (65× 60 = 3900.55× 70 = 3850 3900-3850 = 50,50.

Generally speaking, the special value method is aimed at the complex problem that numbers are difficult to calculate. This method is not necessary for this topic, and it is easier to use the most basic formula of this topic. We can assume that the return speed of this question is x, then we know that the time is equal and get the relationship:100/60+100/x = 200/65 x ≈ 71

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