In a store there are 6 barrels containing 8, 13, 15, 17, 19 and 31 liters of oil or vinegar. We know that a liter of oil costs twice as much as vinegar.
A customer buys € 1400 of oil and € 1400 of vinegar, so that he takes all the barrels except one.
What barrel was left in the store?
The sum of all the barrels gives us 103 liters. We also know that you have bought twice as many liters of vinegar (b) as oil (a). That is to say:
b = 2 a
So playing a little with the numbers, we see that
b = 31 + 17 + 8 = 56
a = 15 + 13 = 28
so that the barrel left in the store was the 19-liter.