A water filtration system for a koi pond needs to be emptied, cleaned, and refilled with new filtration media. According to the manual, it should be filled with 50 lb of a 20% sand / 80% diatomaceous earth mixture. But the store only sells 50% / 50% and 10% / 90% mixtures of sand / diatomaceous earth. Each of these mixtures is sold by the pound, so you can buy as much or as little as you need. Let f be the number of pounds of the 50% / 50% mixture. Which of the following equations matches the scenario?