The mass of a filled vessel is 8 kg. The mass of the same vessel, half full is 5 kg. What is the mass of an empty vessel?

1. We take the mass of the empty container for x, for the mass of the liquid in it.

2. Let’s compose two equations:

x + y = 8;

x + 0.5y = 5;

3. Subtracting the second from the first equation, we get one equation with one unknown:

0.5y = 3;

y = 6 kilograms;

x = 8 – 6 = 2 kilograms or x = 5 – 0.5 x 6 = 2 kilograms.

The answer is the mass of the empty container is 2 kilograms.