Question 1150533
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You would do well to ignore the proclamation by tutor @ikleyn about her mantra.<br>
Find your own mantra.<br>
The solution method shown by tutor @edwin is perfectly valid.  You might find it is more to your liking than hers.<br>
In my opinion, her use of man-days is "irrelevant"....<br>
9 workers is 3/2 times as many as 6 workers; the amount of time required is going to be 2/3 as much.  2/3 of 6 days is 4 days.<br>
Again, ignore anybody who tells you a particular method of solving a problem is wrong.<br>
Keep your mind open and find a solution method that YOU like.<br>