Whether you need the algebra or not, you will probably need to consider the units. We have a tendency not to think in terms of units -- we lump dollars and cents together, for instance, and call it "change."

If you apply an equation without considering the units, you may actually forget they're there. Then your answers will be wrong -- or, perhaps worse, you make the correction on the spot (because you realized what you'd forgotten), but the next person to follow your steps will get the wrong answer and may not know what to do. (The next person may not even realize the answer's wrong.)

What You Can Do

1. Always perform a sanity check! Plug in a reasonable set of numbers first, with their units, make sure the answers make sense, then replace the numbers with algebraic symbols.
2. Use easily recognized, standard units. There's a standard joke among engineers about measuring velocity in "furlongs/fortnight."
3. If there is any chance that your audience or customers are outside the USA, go metric! If you (or others in your organization) aren't comfortable in metric units, solve the problem in English units then convert the results! (This means your mileage varies if you're working with CAD.)

   An example of a situation where metric units won't help is the case of tooling in English units. Then you still have to design for manufacturability.

4. Don't forget that some parameters and variables are invisible. You can't see them among either inputs or outputs. And nearly all unit-conversion parameters fall into this category. The more complex the calculations, the more difficult -- and the more essential -- the bookkeeping.