A christmas string contains 15 lights, 6 are damage, you have to replace them with 6 new lights but don't know which are the damage lights.
1. In how many combinations you can replace the 6 lights.
2. What is the probability that you replace all 6 damage lights.
3. In how many combinations you replace the 6 lights and 5 damage lights are still in the string.
4. In how many combinations you replace the 6 lights and the 6 damage lights are still in the string.