匿名
匿名 發問於 科學及數學數學 · 1 十年 前

Math problems

1. Helen can inspect a case of watches in 5 hours. Jim can inspect the same case of watches in 3 hours. After working alone for 1 hour, Helen stops for lunch. After taking a 40-minute lunch break, Helen and Jim work together to inspect the remaining watches. How long do Helen and Jim work together to complete the job?

2 個解答

評分
  • ?
    Lv 6
    1 十年 前
    最佳解答

    Helen can inspect a case of watches in 5 hours

    i.e. In 1 hour, Helen can inspect 1/5 case.

    Jim can inspect the same case of watches in 3 hours

    i.e. In 1 hour, Jim can inspect 1/3 case.

    Now Helen had worked for 1 hour, that means there are 4/5 case left.

    So, time needed for Helen and Jim work together to complete the job

    =(4/5) / (1/5 + 1/3)

    = 3/2 hours.

    Note: That 40-minute break is no use in calculation.

  • 志仁
    Lv 6
    1 十年 前

    The working rate of Helen and Jim are 1/5 Box/hr and 1/3 box per hour.

    After fisrt hour, 1/5 work done, 1-1/5 = 4/5 work left.

    After lunch, the working rate = 1/5 + 1/3 = 8/15

    Time need = (4/5) / (8/15)=4/5 x 15/8 = 3/2=1.5 hr

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