The phrase "at least" indicates that you use the symbol ≥, so that's where they got the ≥ from. The amount of water needed for each bushel is 1/3 * f or 1/3f because you need 1/3 gallons of water per one bushel. We know that the amount of water needed is at least 1/3 gallons per bushel. Since the amount of water is w, "at least" is ≥ and 1/3 gallons per bushel is 1/3f, the inequality is w ≥ 1/3f. I hope this makes sense.
