There are 6 barrels of root beer in the store.

In the picture below it is shown how much liters of root beer is in each barrel.

2 customers come to the store and one of them bought 2 full barrels, while the other one bought 3.

It is also known that the second customer bought twice as much root beer than the first one.

There was only 1 barrel left, which one?

Need a solution to this.