Suppose you are a farmer. You have 6 barrels.
5 barrels are filled with milk, while the remaining one is filled with water.
Every barrel is filled with an amount (15,19,18,16,20,31). Like pictured below.
You sell the milk barrels to exactly 2 customers.
One customer buys twice as much at the other customer.
How many liters of whater do you have?