Think of how long it would take 5 worker to build a buildind.  How long would it take if they had 5 workers helping them?  How long would it take if they had ten workers helping you? 

The relation between the time it takes to build a building and the number of people working is an inverse proportion.  The more people you have, the less time it takes.

If you double the number of workers, the time is halved.
If you treble the number of workes, the time becomes to a third.

The cross-multiplication method
We are going to apply the same method that we used to solve direct proportions to inverse proportions, but with one additional step to the process: flip over the second column numbers to make it work.