that page you linked to...said "you cannot add rates"....Is there some kind of special circumstance where you are allowed to add rates?
You can't add rates for two different parts, like you can add distances and times. Like "1st leg is 100 mi in 2 hrs & 2nd leg is 300 mi in 5 hrs", but you can't add "20 mph on one street & 40 mph on another street" to get "60 mph for the two streets". That's what they mean when they said "you can't add rates".
But if you're doing combined speeds, like where they're going toward each other so the "net" speed is the combined/added speed, then it's okay. But when it's like that, you can't count both distances or times so "2d" was wrong. It's kind of a context thing I guess.