Let the weight of vessel =v and weight of water =w Then, v+w=40kg‌‌‌‌‌⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(i) and v+‌
w
3
‌‌=20kg 3v+w‌‌=60kg‌‌‌‌‌⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(ii) From (i) and (ii), we get 2v‌‌=20kg v‌‌=10kg Hence, the weight of empty vessel v=10kg