Ken's POTW

A Keg and a Jug
A man had a 10-gallon keg of wine and a jug. One day, he drew off a jugful of wine and filled up the keg with water. Later on, when the wine and water had got thorougly mixed, he drew off another jugful and again filled up the keg with water. The keg then contained equal quantities of wine and water. What was the capacity of the jug?

Source: Henry Ernest Dudeny's 536 Curious Problems & Puzzles

Mail to Ken