I'm interested if anyone has seen something like this urn problem before and if it has a name? The premise is: An urn contains red, green, blue and orange balls. I have a sampling machine that can draw samples from the urn, but will only draw balls of any two colors at a time. For example, the machine can draw red and green balls and I will get a sample that tells me the ratio of red to green but not the proportion of green balls in the urn as a whole. Thanks!