哈，今日又聽到一條看似"mission impossible"的問題。我想這題用英文會好理解一點：
There is a submarine which is located at some integer point on the real axis at the 0th second. It moves with uniform velocity k units / sec, where k is an integer (possibly negative). As the submarine is deep under the water, you cannot see it. Every second you are allowed to fire a missile at some integer point. Find a way to fire the missiles so that you are guaranteed to fire at the submarine.
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哈哈，我想我得到答案了（不過還是遲些才貼出來吧）。你的標籤「countability」給了很大提示呢。
我也有兩個關於 countability 的問題，讓我過幾天在此貼出來。
唔，這個tag我故意留下的線索來的……Dr. Kin Li說還可以有一個「進化版」，不過我還未想到是否可證明。遲一些也可以post出來讓大家討論。
Dr. Li 也問過我這個問題，好像是有學生到Stanford 上summer school 的 "Problem of the Day" 其中一條。
另有一條是這樣的：
Suppose you are blind. You are told that there're 100 coins on the table, and that exactly 20 coins are head. You can flip any coin, but you cannot tell whether a coin is head or tail.
How can you divide the 100 coins into 2 piles such that the number of heads in the 2 piles are the same?
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