A vessel, full of water, weighs 16.5 kg. When the vessel is (1/4)th full, it weight 5.25 kg. Then, weight of the empty vessel (in kg) is?

Let the weight of vesel = x kg
Then, 1/4 of volume of water = (5.25 - x)kg
Full volume of water = (16.5 - x)
∴ 4(5.25 - x) = (16.5 - x)
⇒ 21 - 4x = 16.5 - x
⇒ 21 - 16.5 = 3x
∴ x = 4.5/3 = 1.5 kg