139. Can you measure out 10 liters of water using two containers of 9 and 11 liters?
Solution. Fill the 11-liter vessel (vessel $A$) with water, then use this water to fill the 9-liter vessel (vessel $B$), empty $B$, and transfer the remaining 2 liters from $A$ to $B$. After this, vessel $A$ will be empty, and vessel $B$ will have 2 liters. Repeating these operations three more times, we will end up with an empty vessel $A$ and 8 liters in vessel $B$. Now, fill vessel $A$ from the tap and use it to top up vessel $B$, leaving 10 liters of water in vessel $A$.