1. Hurry up! You have only twenty minutes left.

2. Hurry up! Only twenty minutes are left.

3. Hurry up! You have only twenty minutes (which are) left.
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Are #1 and #2 the same in meaning? Is 'which are' omitted in #2?

All are correct and mean the same.

We don't usually use "which are" in the third sentence.

Yes, sentences #1 and #2 have the same meaning. In sentence #2, the phrase "which are" is omitted. This is a common practice in informal spoken English to make the sentence shorter and more concise. Both sentence #1 and sentence #2 convey the idea that there is a limited amount of time remaining, specifically, twenty minutes.