Let's start with a simple example.  Convert 3 feet to inches.  We already know the  answer, but it's always best to start simple.

So we put what we know into the first box Don't forget to write the units as well, as it will be needed to check your work.  Now the next box will contain a number on top, and one on the bottom.  This is our conversion factor.

It is in case: Now we see that the unit "feet" cancels out.  What are we left with?

Inches.  This was what we were trying to convert to, so we know that is as far as we need to go.  Now multiply out the final answer. 3 Feet = 36 Inches

Example provided by Ask Dr Math

Now, lets try a using one of our physic equation. Final velocity = Initial Velocity + Acceleration x time.  For an accelerating car it has initial velocity is 0 meters per sec; the acceleration is 4 meters per second per second; what is velocity of the car after 20 seconds The term disappears (if you will, it's 0 meters per second).  In the second term, a second cancels from the top and the bottom.  Now multiply out to get your answer. Final Velocity = 80 meters per second