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We did the task in ten minutes.
I am pretty sure that it means
1) It took us ten minutes to do the task. (We spent 10 minutes doing the task)
However, I see no obstacle to suppose that it may also mean
2) We did the task after 10 minutes passed.
(That is, "We did the task in ten minutes after we were told". Hence, we might have done the task in 20 seconds but we had been waiting for 10 minutes before we started doing it.
Do you agree that the second meaning which I infer is possible?
There would be possible ambiguity with, for instance:
We'll do the task in ten minutes.
In that case you might be speaking about how long it will be until you do the task, or the duration it will take you to perform it. You're talking about the future; either is possible.
But your sentence is past tense:
We did the task in ten minutes.
So, no. Your interpretation of "We did the task after 10 minutes passed." is not "possible". "in ten minutes" needs a reference point of "now"...and to be read as "ten minutes from now" you must be using present tense.
No, that interpretation is highly unlikely.
The use of in to mean after some period of time is used in the present, to create a future sense: “I will do that in ten minutes”, “Where do you see yourself in 5 years?”
In the past, you use after as in your sentence 2).
It is interesting to note that in does indeed get interpreted differently, even when we don't use the past tense:
I will do that in ten minutes. -> Ten minutes will pass, and I will start
I can do that in ten minutes. -> Whenever I start, it will take me only ten minutes to finish it
However, the second sentence can also mean you will start after ten minutes, depending on context:
When do you have time for this? Oh, I can do that in ten minutes!
Basically, when in doubt, rephrase your sentence to avoid confusion. When you are confused by what someone says, ask what they mean. :)
The phrase, "in ten minutes", i.e. "in {some length of time}, in isolation, out of context, does have the ambiguity the OP describes. It could mean the length of time something took,
I was able to complete it in ten minutes.
or the length of time something will take:
You should be able to perform this task from start to finish in ten minutes.
or the length of time that must transpire before something will begin:
You can begin the task in ten minutes.
The context determines the meaning of the idiom. As @HostileFork mentioned, the tense of the verb is the disambiguating element of the statement.
