The important piece of information here is that 13 gallons of water makes the container 1/5 of the way full, aka 20%. If the question is simply, how many gallons will it hold, then we multiply 13 by 5 to get 100% at 65 gallons. To get the amount held at 7/10, or 70%, we simply multiply 65 with 7/10 to get 45.5 gallons.
