Converting digits to values using a formula

[vk7]

I have an workbook with 2 worksheets (Sheet1 and Sheet2). Sheet1 contains the summary (Month and Total Expense of the month). The value in Sheet1 (B3) is calculated automatically using a formula. I want the month (A3) to be applied automatically. So in Sheet2 any of these cell ranges (B3:B6 or B8:B11 or B13:16) will be having the month in digits. So if it is 1 then Jan should be added to A3 in Sheet1. Now we have 11 in Sheet2 then it should assign Nov in A3 (Sheet1).

Can someone help me with the formula please? This is my first post, so apologies if I haven't made my question clear. But, I will be happy to provide more details if the requirement is not clear.
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EDIT: by Chihiro at Op's request, removed personal info from workbook and re-uploaded.

 

[vletm]

vk7
Could You use Pivot-table?
.. then You no need to 'solve' months and so on ...
One sample ...

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Edit: by Chihiro - At Op's request stripped personal info from workbook and re-uploaded.

 

[vk7]

Thank you. But unfortunately, I cannot use Pivot table and need to find a formula method.

 

[David Evans]

As @vletm as indicated, why not let Excel handle the date arithmetic for you by way of a Pivot Table, and through restructuring your data by way of an Excel Table - Example attached.
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Edit: by Chihiro - At Op's request stripped personal info from workbook and re-uploaded.

 

[David Evans]

Thank you. But unfortunately, I cannot use Pivot table and need to find a formula method.

If you can't use a pivot table can you use an Excel Table?

 

[vletm]

vk7
... I cannot use ... hmm?
Could You add much more sample data that
it would be more clear how to do it with Your needed way to use 'a formula'?
... and ex are those ranges fixed?

 

[bosco_yip]

Convert number to string,

1] In A3:A100, applied Conditional Formatting, check formula rule as per attached file.

and, if enter 1 to 12 in A3, the cell will assign to "Jan", "Feb"….or,"Dec"

2] In B3, formula :

=SUMIF(Sheet2!$B:$B,A3,Sheet2!$C:$C)

Regards
Bosco
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Edit: by Chihiro - At Op's request stripped personal info and re-uploaded file.