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The picture that comes to mind when thinking about this problem is a stack of pencils with some pencils being sold off.
Remaining pencils = Total pencils - Sold pencils
Remaining pencils = 800 - 257
Remaining pencils = 543
Therefore, Bandile has 543 pencils left.
[Drawing of a stack of pencils with 800 pencils initially, and 257 pencils crossed out to show sold pencils, then 543 pencils left]