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?
- ?Lv 61 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 61 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