[Grammar] for (an) extra two hours

[andi harper]

Hi. I came across this sentence example in Thefreedictionary.com when I was looking up the phrase- balance the books. Here it is "I had to stay at work for an extra two hours last night balancing the books."
My question is why is there an article in the part- for an extra two hours? I think there shouldn't be one. What's everyone else think ? Thanks.

 

[Rover_KE]

I think the dictionary is spot on.

 

[andi harper]

Hi. Thanks for your prompt reply. It's just that I did a Google search without the article and saw it in use too. I'm confused.

 

[kilroy65]

I think that "two hours" is regarded as a period of time (en bloc), hence the singular form and the indefinite article.

Here is a similar example: "Twenty minutes is not enough to finish the job".

 

[Tdol]

I did a Google search for "extra two hours" and virtually every result had the article.