Sharon and Ramla frequently mow their grandfather's yard

题目

Sharon and Ramla frequently mow their grandfather's yard. Working at her normal pace, Sharon can mow the yard in 5 hours. When Sharon and Ramla work together, each at her own normal pace, they can mow the yard in 3 hours. Yesterday, Ramla mowed the yard by herself. Given that Ramla was mowing at her normal pace, how long, in hours, did it take for Ramla to mow the yard?

选项

7.5

8.5

解析

- 莎伦单独修剪院子需要的时间 \( t_1 = 5 \) 小时 - 莎伦和拉姆拉一起修剪院子需要的时间 \( t = 3 \) 小时 - 拉姆拉单独修剪院子需要的时间 \( t_2 \) 小时两人合作完成工作的时间公式为： \[ \text{合作时间} = \frac{t_1 \times t_2}{t_1 + t_2} \] 代入已知条件： \[ 3 = \frac{5 \times t_2}{5 + t_2} \] \[ 15 + 3t_2 = 5t_2 \] \[ 2t_2 = 15 \] \[ t_2 = 7.5 \] 选项：B

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