6. Mary had 3/4 cup of juice in her glass. She drank 1/3 of it. How much juice is left?

What does it refer to? It refers to the juice. This is saying:

Mary had 3/4 cup of juice in her glass. She drank 1/3 of the juice. How much juice is left?

This problem is of exactly the same form as the M&Ms and the plants. The distinction is between 1/3 of the juice and 1/3 of a cup of juice (you may assume that you are measuring 3/4 and 1/3 with respect to a measuring cup or with respect to the cup that her juice is in--the problem gives the same answer either way).

Original problem: Mary had 3/4 cup of juice in her glass. She drank 1/3 of it. How much juice is left?	She drinks 1/3 of the juice which is 1/3 of 3/4 of a cup. Mary drinks (1/3x3/4)=1/4 of a cup of juice Solution: 1/2 cup is left
Simple subtraction problem: Mary had 3/4 cup of juice in her glass. She drank 1/3 of a/the cup of it. How much juice is left?	She drinks 1/3 cup of the juice Solution 3/4 - 1/3 = 5/12 cup is left

Notice here, as in the plant problem, that our natural tendency is to give the fraction to be taken away as a fraction of the amount we are taking away from, which changes the whole. In order to add or subtract the wholes must be the same size.