Unit analysis means using the rules of multiplying and reducing fractions to solve problems involving different units.
A desk is feet long. Find the length of the desk in inches.
There are inches in foot. Write this relationship as a fraction, and multiply it by the given length.
Note that you can cancel the unit "ft" just as though it were a number being multiplied.
A car can drive miles on worth of gasoline. If gallon of gasoline costs , find how far the car can travel in kilometers on liter of gasoline.
To solve this problem you need to know some conversions.
Write all the conversions as fractions, so that the unwanted units will cancel.