Barrel of beer
A man bought 6 barrels of booze. 5 contained wine and one contained beer. The sizes of the barrels were 15, 16, 18, 19, 20 and 31 gallons (luvvly jubbly). He sold a number of barrels of wine to one customer, and later sold some more barrels of wine to another customer. The second customer bought twice as much as the first.
How much beer was in the beer barrel?
(Use brain, not brawn, for this one.)