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From statements (i) and (ii) we can infer that the black ball is heavier than the red, blue and yellow balls since the yellow ball is lighter than the red ball and the black ball is heavier than the red ball.
Similarly from statements (i) and (ii), we can conclude that the black ball is heavier than the green ball. Therefore, the black ball is the heaviest.
Now from statements (ii) and (iii), we can infer that the blue ball is the second heaviest.
It is given that the yellow ball is lighter than the red ball so the red ball cannot be the lightest ball.
Therefore the lightest ball is either the green ball or the yellow ball.