The cost of sending a package depends on its weight. A service charges $5 for each pound of weight for standard shipping and $8 for each pound for express shipping. The equation $5S+8E=50$ represents the total cost, in dollars, for shipping $S$ pounds using standard shipping and $E$ pounds using express shipping. If a customer spent $50 to ship a package using 2 pounds of express shipping, how many pounds were shipped using standard shipping?