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Not particularly hard if you think recursively. In case of a single blue eyed person, he would see 199 people with other eye colours, telling him the colour of this own eye, causing him to leave on the first night. This is the base case. If he doesn't leave, there is at least one more person with blue eyes, causing the second blue eyed person to realize that he is the other blue eyed person on the second noon, as he sees that there are 198 people with other eye colours, letting them leave on the second night. Repeat until all blue eyed people realize their eye colour