Professor Zhao from the Mathematics Department suddenly noticed her granddaughter, Xiaoyun, calculating in a strange way: she used her thumb to represent 1, index finger to represent 2, middle finger to represent 3, ring finger to represent 4, and little finger to represent 5; then she went back, using the ring finger to represent 6, middle finger to represent 7, index finger to represent 8, and the thumb to represent 9; after that, she continued, with the index finger representing 10, the middle finger representing 11, ... , and when she reached 20, she stopped exactly on the ring finger.