Brain Teasers and Puzzles

A     B
9     0          fill A
5     4          fill B from A (leaving 5 litres in A)
5     0          empty B
1     4          fill B from A (leaving 1 litre in A)
1     0          empty B
0     1          fill B from A
9     1          fill A
6     4          fill B from A (only 3 more litres fit in B)
6     0          empty B