MHB Man-Days and Man-Hours: Understanding Units of Work Measurement

AI Thread Summary
The discussion revolves around calculating the time it takes for one of the faster men to complete a job alone, given that two of the men work three times faster than the third. The initial calculation suggested it would take 20 days, but the correct answer is 28 days, derived from understanding the work rates of the men involved. The concept of "man-day" is clarified as the amount of work one man can accomplish in a day, which is often confused with "man-hours," a broader measure of work completed. The conversation highlights the importance of accurately defining these terms to solve work measurement problems effectively. Understanding these units is crucial for clear communication in project management and labor estimation.
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If 3 m e n do a job in 12 days and two of the me n are three time s as fast as the third,
how long will it take one of the faster men to do the job ?

this is how I solved it

let x = required time for one of the faster men to do the job alone
3x = time taken by third to finish a job alone.

$\frac{1}{3x}+\frac{1}{3x}+\frac{1}{x}=\frac{1}{12}$

x = 20 days

is this correct?
 
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No, that's not correct. The way I see it, if would take the slow man $12\cdot7=84$ days to complete the job alone, so one of the faster men would take $$\frac{84}{3}=28$$ days.

You would find this also by using:

$$\frac{1}{3x}+\frac{2}{x}=\frac{1}{12}$$

$$\frac{7}{3x}=\frac{1}{12}$$

$$\frac{3x}{7}=12$$

$$x=28$$
 
$12\cdot7=84$ where did you get this?
 
The two faster men can do the work of 6 slower men, so having the two faster men and one slow man working is equivalent to 7 slow men working.
 
markFL, I saw a solution to this that uses "man-day" which until now I don't have a complete understanding. can you explain about it?

some say that man-day is the work-rate of a single man in one day
some say that it is the amount of work a single person need to complete a job.
some say it is a work-load. which confuses me all the time I encounter these types of problems.

I'm hoping that you could show me the explanation about it. Thanks!
 
Simply put, a man-day is the amount of work one man can get done in one day.

You may have heard a more commonly used unit of man-hours. For example, it may be reported to have taken 2,000 man-hours to complete the design of the PCM for the latest hypercar. This may mean 1 man working for 2,000 hours or 2,000 men working for 1 hour, or 4 men working an average of 500 hours each, etc.
 
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