15. (6 points) A three-digit number can be simultaneously divided by $4, 5, 7$. If such three-digit numbers are arranged in ascending order, the middle number is $\qquad$ .
【Answer】Solution: Since the least common multiple of $4,5,7$ is $4 \times 5 \times 7=140$, the three-digit numbers that meet the criteria are $140,280,420,560,700,840,980$, so the middle number is 560. Therefore, the answer is: 560.