Dimensions and Units
Every expression you write down that tries to describe something about the real world has to be dimensionally consistent. After all, what does the expression 5mi hr +3ft mean? Or an equation such as 6sec−2km = 4kg? Or the statement that the distance from here to downtown Miami is 8? Answer: Nothing! (Well, maybe in the last case there is a context, and you know that the speaker meant eight miles, but often you don't know this, and it means there's a mistake.)
You can figure out correct units for something just by solving a little algebra.