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Assuming you are asking what does x represent when you say "What does represent" the answer is a: the number of days it take the brothers to paint 13 pictures.
The first 2 sentences are useless and don't tell you anything (well they do but you don't need the information).
Both brothers can paint 13 pictures in x days.
So x means days, more specifically, the number of days it took for the brothers to paint 13 pictures.
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