Parallel rates add; sequential rates don't

It makes sense to add parallel rates. For example, when filling a bathtub with 2 taps, you can sum the rates of both taps and get the correct rate of filling

$$5L/min+3L/min=8L/min$$

It does not make sense to add rates when in sequence. When a single car goes 60km/h some distance, then 30km/h back the same distance, you can’t add those like:

$$60km/h+30km/h\ne 90km/h$$