6. Using the four digits 6, 7, 8, and 9, many four-digit numbers without repeated digits can be formed. If they are arranged in ascending order, 9768 is the  th.
【Analysis】The four digits $6,7,8,9$ can form $4 \times 3 \times 2 \times 1=24$ (numbers) without repeating digits. Observing the number 9768, which is relatively large, and arranging it from largest to smallest, it is found to be the 4th number. Therefore, when arranged from smallest to largest, 9768 is the 21st number.