Because 4 gallons get drained every minute you need to find the difference in 50 1/2 and 26 1/2 which is 24, so 24 gallons need to be drained until the amount of water is 26 1/2. So 24 gallons will drain in 6 minutes as 4 gallons drain every minute. It will take 6 minutes for the amount of water to be 26 1/2