The doctor orders Ceclor elixir 25 mg/kg for a child who weighs 20.4 lbs. The available Ceclor elixir contains 250 mg per 5 ml. A nurse must determine the proper dose for the child.

The nurse converts the child's weight from standard pounds (20.4 lbs) to metric kilograms (9.26 kg) and uses a calculator to quickly calculate a dose of:
.

This number is clearly too large for a drug dose, so the nurse must check the calculation by rewriting the equation with units rather than just with numbers. With units, the calculation just completed appears as follows:
.

Since the units in the calculator version are not ml's, as they should be, it is easy for the nurse to identify the mistake: The last term (mg/ml) should have been inverted (ml/mg). So the nurse must repeat the calculation to obtain the correct dosage: