Dimensional analysis
Any math done with numbers must also be done with their unit labels.  For example, when dividing 50 miles driven by 2 gallons of gas used, you not only divide 50 by 2 to get a result of 25, you also have to divide "miles" by "gallons", which gives the unit "miles/gallon" or "miles per gallon".  The final answer is "25 miles per gallon".