Builders and days are inversely proportional because the more builders the 
less the time, and the fewer builders the more the time.  So you must be careful
in setting up the proportion:

Just always remember that when you set up a proportion, if you put a larger
number over a smaller number on the left side, you MUST also put a larger number 
over a smaller number on the right side as well.

Even though you don't know at the start the value of the unknown, you can
always tell at the start whether it is going to be larger or smaller than
other corresponding number.  So you can always set the proportion up 
correctly.  

 





Cross-multiply:



 

Edwin