1)fill 5 liter cup and then pour those five liters into 8 liter cup and again fill 5 liter cup
We have
12 lt jar having 2liters milk,
8 lt jar having 5 liter and
5 lt jar having 5 liter

2) Then pour 5liters milk into 8 lt jar so that 2lts left in 5 lt jar
now
12 lt jar have 2 lt  milk
8lt jar  have 8 lts milk and
5lt jar have 2 lts milk

3) Then pour 2 lts milk of 5 liter jar into 12liter jar and pour 8 liters into 5 lts  so that 3 liter left in 8 liter jar
now
12 lt jar have 4 lt milk and
8 lt jar have 3 lt milk and
5 lt jar have 5 lt milk

4) Pour 5lit into 12 liters so totally 9 liters left in 12 lt jar
now,
12 lt jar have 9 lt milk and
8 lt jar have 3ltr milk  and
5 lt jar have 0ltr milk

5) Pour 3ltr milk of 8 lt jar into 5jar and pour 9liter of 12 jar into 8 so that 1 liter left
12lt jar have 1 lt milk and
8 lt jar have 8lt milk  and
5 lt jar have 3lt milk

6) Pour 2lt milk from 8Lt jar into 5lt jar to fill it completely
Finally,
12 lt jar have 1 lt milk and
8 liters jar have 6 liters of milk and
5 lt jar have 5 lt milk

So, we got 6 liters milk

elephant