How to divide equally between two families 12 y. bread kvass in a 12-liter vessel, using two empty vessels: 8 liter and 3 liter?

First, you should take a 3 liter vessel of kvass and pour it into an 8 liter vessel.
Thus, in an 8 liter volume there will already be 3 liters of kvass.
Then you should again take a full 3 liter vessel and pour its content into an 8 liter vessel of kvass, where there are already 3 liters of kvass.
In this case, it will already contain:
3 + 3 = 6 liters of kvass.

After this procedure, the following will also remain in a 12-liter vessel of bread kvass:
12 – 3 – 3 = 6 liters of kvass.

Thus, both families will receive 6 liters of kvass.