"Tardé unas pocas horas en hacerlo" is right as well, isn't it?

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Hola Robert
Using "unas pocas horas" would change the meaning. 
What we have in the test question is:
Tardé pocas en hacerlo. 
It didn't take long to do it. [lit: I took few hours to do it.]
This means that you didn't take long, so you only took 2 or 3 hours for example, not many.
However is you say:
Tardé unas pocas en hacerlo
what you mean is "It took me a few hours" with the implication/nuance of taking a long time. 
Saludos
Inma

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This helped - I was struggling with the same thing - the difference between pocas and unas pocas.Thank you, Inma!