Answer:
The stones described, which are radiolucent with 10-20% opacity due to calcifications and are found in the gallbladder (cholelithiasis), are most likely composed of mixed materials, including cholesterol and calcium bilirubinate. These types of gallstones are called mixed stones. The cholesterol component makes them radiolucent, while the calcium bilirubinate component contributes to the calcifications that give them some opacity on imaging.