Formulas and unit analysis
Rearrange the formula for the unknown before substituting, and cancel units through the working to check the answer.
A worked example
A road rising 3 m over 60 m of run has a grade of 3/60 = 0.05, or 5%. And 72 km/h x 1 000 m/km ÷ 3 600 s/h = 20 m/s.
The mistake people actually make
Percent grade is rise divided by horizontal run, not by the sloped length. In I = Prt the time is in years and the rate goes in as a decimal.