Answers:

1. The 8th Day (At the end of Day 7, it is at 7 metres. On Day 8, it climbs 3 metres to reach 10 metres and finishes before it can slide back).

   
   

2. 25cm2

   
   

3. 11 posts

   
   

4. 150 kg

   
   

5. 9 sides (a nonagon)