Grain Weights and Sacks Calculation

A farmer has three types of grains: wheat, barley, and corn. Each type has a specific weight per kilogram when measured in sacks.

- A sack of wheat weighs 2.5 kilograms.

- A sack of barley weighs 1.8 kilograms.

- A sack of corn weighs 3.2 kilograms.

The farmer has a total of 15 sacks, comprising some combination of these three types of grains. If he has twice as many sacks of corn as he does of wheat and the total weight of all the grains is 30 kilograms, how many sacks of barley does he have?