Question 1071135
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The equation for man-days is


6*15 = 10*x.   (1)


If you want to write it as a proportion, here it is:


{{{6/x}}} = {{{10/15}}}.


Inverse proportionality.


If you want, you can present (1) equivalently as the equality for the rates:


{{{1/(6*15)}}} = {{{1/(10*x)}}}.     (2).


Both sides of (2) are rates (that are assumed to be the same).
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To see solved problems on this subject, look into the lesson

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Rate-of-work-word-problems/Rate-of-work-problem.lesson>Rate of work problems</A> 

in this site.



Also, you have this free of charge online textbook in ALGEBRA-I in this site

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson>ALGEBRA-I - YOUR ONLINE TEXTBOOK</A>.


The referred lessons are the part of this textbook under the topic "<U>Rate of work and joint work problems</U>" of the section "<U>Word problems</U>".