15. (3 points) A tree is 24 meters high, and at the bottom of the tree, there is a snail that wants to climb from the bottom to the top of the tree. It climbs up 6 meters during the day and slides down 4 meters at night. This snail will take $\qquad$ days to reach the top of the tree.
【Answer】Solution: Each day actually climbs: $6-4=2$ (meters), (24-6) $\div 2=9$ (days), so it takes $9+1=10$ (days). Answer: The snail can climb to the top of the tree in 10 days. Therefore, the answer is: 10.