If someone says:
The time it
took me to walk to class today was 10 minutes.
the number could
possibly be wrong, but the statement
at least makes sense.
If someone says:
The time it
took me to walk to class today was 10 kilograms.
something is
obviously wrong.
When you use a formula to calculate some answer you can treat units just like numbers multiplying and canceling them.
The units you are left with MUST be those which match the ones expected or you have made some mistake.