Greetings. Firstly, my apologies if this is in the wrong place but this seemed to be the most appropriate board. I am not a Physics student but my friend, who is, posed me the probability question below in order to teach me about quantum probability and how it differs from classical probability: You have a box and inside the box are two shapes. The first shape has a 50% chance of being a cube and a 50% chance of being a tetrahedron. The second shape also has a 50% chance of being a cube and a 50% chance of being a tetrahedron. You also know that any cube in the box (if indeed there is a cube in there) could be yellow. Question: What is the probability that at least one of the shapes in the box is a cube? Now my initial response was to think that seeing as I know the probabilities for both the shapes in the box as to whether they're cubes or not, the information I'm given about the possible colour of any cubes in the box is completely irrelevent. Thus I have quite a simple problem and I quickly arrived at the answer of 3/4. However, according to my friend, the answer is 7/8. Apparently, according to the rules of Quantum probability theory, I am allowed to deduce from the information given that any tetrahedron in the box can't be yellow and the answer follows from this. However, I didn't understand any more of his explanation as to why this was so. I'm now burning with curiosity to understand and I thought that a Physics forum would be a good place to go to seek enlightenment! So would someone be able to explain to me what's going on? Thank you.