sat数学题

wayne3721UID: 2745699

A jar contains 20 marbles:4 red, 6 white, and 10 blue, if you remove 1 marble at a time randoly, what is the minimum number that you must remove to be certain that you have at least 2 marbles of each color? 答案是18

我知道这是概率问题，但是我不理解的地方是，既然说是minimum number。为什么不是6次而是最大的18次呢？还是minimum这个词本身就是指最小可能性的意思呢？