Unit conversions are very useful in the sciences and all types of engineering. I first learned unit conversions in 9th grade, and have used them since, all throughout college and at work. This is how it works:

To convert 80 miles/hour to meters/second:

 80 miles |    1 hour   |  1 minute  | 5280.01056 feet | 0.3048 meters |
----------|-------------|------------|-----------------|---------------| = 35.76 meters/second
   hour   |  60 minutes | 60 seconds |     1 mile      |    1 feet     |   

All of the units in the numerators and denominators cancel out except for those desired. Of course, I used a handbook that had standard measurements like 1 pound = this and 1 btu = that, as in SI Units.