If a 12 gallon tank is filled with water at rate of 1/4 of the tank in 5 seconds, then it's filled with 3 gallons per 5 seconds (because 1/4 * 12 = 3). 5/6 of the tank is as much as 5/6 * 12 = 5 = 60/6 = 10 gallons.
If 3 gallons of the tank are filled in 5 seconds, then 10 gallons will be filled in (10 ÷ 3) * 5 = 3 1/3 * 5 = 16 2/3 seconds.
Answer: It will take 0 minutes and 16 2/3 seconds to fill 5/6 of the tank.
I hope that's what you meant :)
