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To work the Rule of Three inverse, or the back Rule of Three by the Lines of Lines.
In these proportions there are alwayes three terms given to finde a fourth, and of the three given terms two are of one deno∣mination (which for distinction sake I call the double denomination;) and the third term is of a different denomination from those two, which I therefore call the single denomination, of which the fourth term sought must also be. Now to bring these
into a direct proportion, the rule is this. When the fourth term sought is to be greater than the single denomination (which you may know by sight of the terms given) say,
As the lesser double denomination is to the greater double denomination, so is the single denomination to the fourth term sought. The work is by Probl. 2.
If 60 men do a work in 5 dayes, how long will 30 men be about it?
As 30 is to 60, so is 5 to 10. The number of dayes for 30 men in the work.
Again, when the fourth term is to be less than the single denomination, say,
As the greater double denomination is to the lesser double denomination, so is the single denomination to the fourth term sought.
If 30 men do a work in 5 dayes, how long shall 60 be doing of it?
As 60 is to 30, so is 5 to 2½. The time for 60 men in the work.