Dan wants to make trail mix made up of almonds, pecans, and raisins. He wants a mix of one part almonds, two parts pecans, and three parts raisins. Almonds cost $12 per pound, pecans cost $8 per pound, and raisins cost $4 per pound. Dan has $30 to spend on the trail mix. Which equation can be used to find how many pounds of trail mix he can buy for $30?