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in Example 18: Two women, Sujain and Mariya, working separately can mow a field in 8 hrs and 12 hrs respectively. If they work in stretches of one hour alternately, Sujain beginning at 9 a.m., when will the mowing be finished?-----------------------
in 2hrs 5/24 of field is mowed so in one hr 5/48 of field is moved. in 9hrs 45/48 or 15/16 of field is moved. so time taken to mow 1/16th of field =1/16 by 1/12 hours or 12/16=3/4hours or 45mins. I understood your method, but what is wrong in this method? there is a difference of 15mins!
Answers (1)
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Actually what you are doing is that you are calculating the average work done in 1 hr.
what you have done is that you have calculated the work done in 2 hrs = 5/24 and then divided by 1/2 to get the work done in 1 hr.
Now if i ask you how much work will be done in 3 hrs , then your answer would be 3 x 5/48 = 5/16
But in reality , the amount of work done in 3 hrs = 5/24 + 1/8 = 1/3
This is because in your approach you have assumed that the work done per hr is equal i.e. 5/48 + 5/48 + 5/48
whereas in reality it is 1/8 + 1/12 + 1/8 , it is not the same in every hr.
we hope you have understood your mistake.