Answer:
1/2 (1 half)
Explanation:
The number of different sandwiches is calculated multiplying all of the possibilities for each material used:
rye or white bread: 2 options
ham or turkey: 2 options
cheese or no cheese: 2 options
So the number of different sandwiches that can be made is 2*2*2 = 8.
From these 8 different sandwiches, 4 have cheese and 4 have no cheese, as the staff made a equal number of each type of sandwich.
So, if from 8 different type of sandwiches, 4 have cheese, the chances of Mary getting a sandwich with cheese is 4/8 = 1/2 (1 half).