1)There is a string of 10 Christmas lights in a row. Suddenly, 3 random lights burn out. What is the probability that no two burnt out lights are next to each other?
2) Dylan, Helen, Joseph, Kenny, and Mike are taking photos together in a straight line to celerate their graduation. Mike wants to stand next to Helen, and Dylan does not want to stand beside Kenny. Find the number of ways to take the photo.
3) 1/3 + 1/4 +1/5+1/6............+1/25+1/26
求答案和方法。谢谢!!!