At a time means in one go, together: only three visitors can enter at a time. One at a time means one after another, never two together. With a length of time, it means per session: she works on the model for twenty minutes at a time.
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one / two / a few at a time; an amount or duration + at a time
- Only three visitors can enter at a time.
- Upload one file at a time so the names are easier to check.
- She works on the model for twenty minutes at a time.
What changed
First versionPlease bring the boxes two at time.
RevisionPlease bring the boxes two at a time.
Numbers often go straight before a word with no article, so a drops out: two at time. But the phrase needs a: two at a time. Without it, the sentence sounds clipped.
At times means sometimes: the work is boring at times. At the same time means simultaneously, with no number. One by one means one after another; one at a time stresses the limit: no more than one.
Your turn
You want people to enter in groups of two: Please come in two ___.
Show the answer
at a time — People should come in groups of two, so it's two at a time. At times means sometimes.
Write a short instruction saying how many people or things are allowed at once and why, using at a time.
Scan five pages at a time so the tray does not become overloaded.
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