In the kitchen in your apartment, you put all your 20 forks in the left drawer and all 20 knives in the right drawer. Your roommate, who does not agree with your organizational approach, comes in, takes five forks from the left drawer and tosses them into the right drawer. She then takes at random an item (knife or fork) from the right drawer and tosses it in the left drawer. After this exchange, you come in and randomly pick up an item from a randomly chosen drawer. Given you have picked up a knife, what is the probability that you have opened the left drawer?