The method of finding the 4th proportional when the other three are given is called Simple Proportional or Rule of three.
Repeated use of three is called Compound Proportional.
Two quantities are said to vary directly if on the increase ( or decrease) of the one , the other increase (or decrease) to the same extent.
More articles , more cost.
More men, more work.
Two quantities are said to vary indirectly if on the increase ( or decrease) of the one , the other decrease (or increase ) to the same extent.
Less time to finish a work , more persons at work.
More speed , less is the time taken.