When 'tarda....en' = 'it takes...'

Question

I keep tripping up on this! Asked to choose correct answers in a multiple choice, I chose as follows:

It takes 10 minutes to paint it = Se tarda..., Se tardaron... and Tarda...

Tarda was marked as incorrect. However, it seems to me that it has some legitimacy. Thing is, there's no indication what 'it' actually is, so it could in fact be the subject of the sentence! That might seem odd, but perhaps if you were to think of 'it' being a train in the sentence '(el tren) Tarda dos horas en llegar a Madrid' it might make a bit more sense - especially if this was said as a reply and the subject was already established.

If the question gave a hint like 'form a passive' or 'we don't know who or what the subject is', that would leave little doubt. But as is, it not entirely clear.

Answer

Hola Graeme,
In this sentence:  "It takes 10 minutes to paint it" as it states in the quiz, I can't see any room for a definite subject that has been omitted. This is an impersonal sentence with no subject which needs an impersonal form using "se", either "se tarda" or "se tardan". 
I agree that in the case where the subject could be omitted because it is understood by a previous statement, for example:

Este tren es rapidísimo. Tarda 1 hora en llegar. 
This train is very fast. It takes 1 hour to get there.

We are clearly omitting "El tren ..." in the second sentence because it is not necessary to repeat. But this is not same as in the sentence from the quiz. Even if you think that there is an omitted subject, the only possible subject (because of the meaning of the sentence) would be a person. If that was the case, that we were omitting a known subject, being a person, the sentence would be: 
He/she takes 10 minutes to paint it. 
or 
It takes him/her 10 minutes to paint it.
But we are saying:
It takes 10 minutes to paint it ( = one takes 10 minutes to paint it - impersonal)
I hope this clarified it.
Inma