SOLUTION: A jug filled with water weight 2kg 450g. When the jug is 1/5 filled with water, it weights 1.05 kg what is the mass of the jug when it is half filled with water?

J +    W = 2450,    (1)   (J = jug's weight;  W = water weight when jug is filled).
J + 0.2W = 1050.    (2)

To solve the system, distract (2) from (1). You will get

0.8W = 2450 - 1050,   or

0.8W = 1400  --->  W =  = 1750 grams.

Thus water weights 1750 gram when the jug is full.

Now, the answer to the problems is 2450 - 

(jug filled with water minus half of 1750 grams of water).

Can you calculate it on your own?