You were on the right track! The idea is to make the denominators the same, and depict it in a model to make things look simpler. Since 2/5 of the items were keychains, 2/3 of 2/5 = 4/15. The question already said that the items left make up 1/3, so by making the denominator the same you get 5/15.

1-(4/15)-(5/15) = 6/15, which is equal to the 48 bookmarks sold. From there, you can get 1/15 = 8, and 15/15 = 120.