400. Milk Delivery. One morning, a milkman was transporting two 80-liter barrels of milk to his shop when he encountered two women who begged him to sell them 2 liters of milk each right away. Mrs. Green had a 5-liter jug, and Mrs. Brown had a 4-liter jug, while the milkman had nothing to measure the milk with.

How did the milkman manage to pour exactly 2 liters of milk into each jug? The second portion posed the greatest difficulty. However, he successfully solved the problem in just 9 operations. (By "operation," we mean pouring either from the barrel to the jug, from one jug to another, or, finally, from the jug back to the barrel.)

How did the milkman proceed?
400. The simplest solution to the problem is as follows (the capacity of the vessels is indicated above, the initial amount of contents below, and in each subsequent line - the amount of contents after each operation):

\begin{tabular}{llll}
80 liters & 80 liters & 5 liters & 4 liters \\
80 & 80 & 0 & 0 \\
75 & 80 & 5 & 0 \\
75 & 80 & 1 & 4 \\
79 & 80 & 1 & 0 \\
79 & 80 & 0 & 1 \\
74 & 80 & 5 & 1 \\
74 & 80 & 2 & 4 \\
78 & 80 & 2 & 0 \\
78 & 76 & 2 & 4 \\
80 & 76 & 2 & 2
\end{tabular}

Thus, we first fill the 5-liter jug from one of the barrels, then the 4-liter jug from the 5-liter jug, then pour the contents of the 4-liter jug back into the barrel, and so on. All of this can be done very easily. Note the ingenuity of the last two operations: we fill the 4-liter jug from the second barrel, and then top up the first barrel to the brim.