The cafeteria creates pre-made boxed lunches with equal numbers of the following items:

• a sandwich made with either white or wheat bread and either roast beef or bologna
• a snack that is either chips, popcorn, or pretzels
• a drink that is either bottled water or juice

If Gretchen randomly chooses one of the boxed lunches, what is the probability that she will get a roast beef sandwich on wheat bread and popcorn in her box?