Which box is the

Solution

Box G is heavier than at least two boxes. Box J is heavier than box I and box H but not the heaviest. Number of boxes heavier than box J is equal to number of boxes lighter than box K. Box H is heavier than box G. Box H is not the 3rd heaviest box. Box M is heavier than box L which is lighter than box K.

So, box J and box K must be 2nd heaviest and 2nd lightest boxes respectively. Also, As, box H is not the 3rd heaviest, so, I must be 3rd heaviest and M must be heaviest box.

The arrangement is shown below,

M > J > I > H > G > K >. L