This is what is called dimensional analysis. Multiply by the unit factors to convert units. You can multiply and divide units just like numbers, set up the problems so the units cancel (divide out) leaving the desired units.
Example
[tex]25 m * \frac{100cm}{1m} * \frac{1in}{2.54cm} * \frac{1ft}{12in} = 82 feet[/tex]
see how the you multiply by the first factor cancels out meters, changes to cm. Then the next factor gets multiplied cancels out centimeters and changes the units to inches. The last factor is multiplied which cancels out inches and changes the units to feet.
