Each of the five balls, such as Red, Blue, Yellow, Black and Green is of a different weight. It is known that:
(i) Black ball is heavier than the Red and Blue balls.
(ii) Yellow ball is lighter than the Red ball.
(iii) Blue ball is heavier than the Red and Green balls.

Which ball could be the lightest one?

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.