The horse will receive his full portion of water from the container during 8 (eight) days and 1 (one) gallon is left.
If the horse drinks 7 gallons each day, so in 8 days, he will drink 56 gallons.
As the container has a capacity of 57 gallons, the horse will receive his full portion during 8 days.
The trainer has to refill the container of water on the 9th day.
