.
The equation for man-days is
6*15 = 10*x. (1)
If you want to write it as a proportion, here it is:
= .
Inverse proportionality.
If you want, you can present (1) equivalently as the equality for the rates:
= . (2).
Both sides of (2) are rates (that are assumed to be the same).
To see solved problems on this subject, look into the lesson
- Rate of work problems
in this site.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this textbook under the topic "Rate of work and joint work problems" of the section "Word problems".