To save on helium costs, a balloon is inflated with both helium and nitrogen gas. Between the two gases, the balloon can be inflated up to 8 liters in volume. The density of helium is 0.20 grams per liter, and the density of nitrogen is 1.30 grams per liter. The balloon must be filled so that the volumetric average density of the balloon is lower than that of air, which has a density of 1.20 grams per liter. Which of the following system of inequalities best describes how the balloon will be filled, if x represents the number of liters of helium and y represents the number of liters of nitrogen ?