Assuming we're talking about VB6 here, the simplest way to tally up the numbers in a list box would to use a loop something like...
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For x = 0 to listbox.ListCount - 1
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Tally = Tally + Val(listbox.List(x))
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Next
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Note the odd start/end values for the loop, thanks to the silly zero-based index that MS decided to use.