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Explanation:
If he gave 3 markers, and it means he has 13 markers left, that must mean he started with 13+3 = 16 markers overall. Divide that count among the two boxes to get 16/2 = 8 markers per box.
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Using algebra, we can say,
x = number of markers per box
2x = number of markers total before he gives his sister any markers
2x-3 = number he has after giving the 3 markers
2x-3 = 13
2x = 13+3
2x = 16
x = 16/2
x = 8
