Let x = liters in a full tank.

To say the tank is 3/5ths fulls means there are (3/5)x liters of fuel.

To say the tank is 9/20ths full means there are (9/20)x liters.

Can you use these expressions, to form an equation, based on the given information that using 6 liters of the first amount leaves the second amount? :cool: