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Tom: Hello! May I speak to Mary?

John: She isn't here right now.

Tom: When do you think she will be back?

John: She must/will be home around six or so. (I'm trying to express "It is very likely that she will be around six or so." with "must", Is that possible?)

Please give me your opinion about this, thank you!
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Hi Viceidol, we'd use "should" in this spot. I've never heard "must" used in this particular case.

Best wishes, - A.
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Must can be used with the meaning of very likely only for the past, in "must have" combinations:

I couldn't find her home. She must have left for some errands.
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Comments  
ViceidolI'm trying to express "It is very likely that she will be around six or so."
It can't be done with must.
She must be home around six means that she is required to be home around six, that she has to be home around six.
CJ
 Marius Hancu's reply was promoted to an answer.
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Or another thought totally different from Viceidol's original: She must be home by midnight or she'll never go out with you again, young man!