Reflective Shield

Question: A shield can reflect projectiles which travel at 4/5 of their original speed after being reflected. A missile is shot towards the shield at 700 km/h. At what speed did it go after its reflection?

Question: Surrounded by life , yet life is its biggest enemy. But without life there is no need for it to exist. What is it?

Question: During what month do people sleep the least?

Question: When is 99 more than 100?

Question: People see through me, but none pass through without killing me. I can be anywhere, and can be seen double or single. What am I?

Question: The more you add, the less it weighs.