Trivial Example Two ....

We're on that same drive, but now Bob asks how far we've gone in 30 minutes.

We still start with what we want to know - and that's still how far or miles:

 

 

Then take the thing we do know writing it so the unit is in the same position:

 

 

           

but now when we factor in the other thing we know, we have a small problem: the other thing we know is 30 min and that isn't the same unit as hr, so they aren't going to cancel out.

 

 

What we need here is another factor that lets us get rid of the hours and the minutes. And that of course is a conversion factor. It's useful to remember that when we use a conversion factor what we are really doing is multiplying by one. Click on conversion factor if you want to see how that works. The important part is that since all we're doing is multiplying by one, we can always multiply by a valid conversion factor - or for that matter any number of them because we haven't really changed anything.

 

 

Now when we cancel :

 

 

we get :

 

 

This might seem like a lot to do just for an answer that you do in your head without even thinking about it. The point is we DO know how to do problems like this - what we don't usually think about is HOW we do them. And the HOW is what this factor-label stuff is all about. Lets look at one more example before we move on to the nursing math ......

 

©1997-2006 Dale Sampson, RN