Learn Russian: Почему автобус номер восемьдесят едет так медленно? - Why is bus number eighty driving so slowly?

It is a compound number made of восемь (eight) and десят (an old form of ten). It literally means 'eight tens'. Notice that the soft sign (ь) stays in the middle of the word, which is the spelling rule for 50, 60, 70, and 80.

Why do we say номер восемьдесят (number eighty) instead of using an ordinal word like 'eightieth'?

When talking about transport routes, Russians often say номер followed by the regular counting number, just like in English. You could also say восемьдесятый автобус (the eightieth bus), but the phrasing in this sentence is very common.

Why use the verb едет here instead of идёт or ездит?

Едет is used for traveling by a vehicle in one specific direction right now. Since the bus is currently driving along its route, this is the perfect verb. Идёт is mostly for walking, and ездит would mean the bus drives this route regularly, rather than focusing on its current slow progress.

Why is it так медленно and not такой медленный?