Deshaun’s gas tank holds 20 gallons of gas.
6 gallons + x gallons = 40% of the tank’s capacity.
6 gallons = 30% of the tank’s capacity.
That means that 18 gallons = 90% of the tank’s capacity.
10% of the tank’s capacity = 2 gallons.
Therefore, 100% of the tank’s capacity is 20 gallons. Meaning Deshaun’s gas tank holds 20 gallons of gas.