There are 54 cups of juice from one bottle.
54/54 makes one bottle of juice.
However, you are trying to find out how much bottles are needed to fill one cup.
54 - 1 = 53
Place 53 over the total amount (54)
53/54 is the amount not used
subtract it from 54/54
54/54 - 53/54 = 1/54
1/54ths of a bottle is used to fill one cup
hope this helps