A magician asked the audience to write down a six-digit a, and add the digits of a to get b. He asked the audience to say the 5 numbers in ab, and the audience reported 1, 3, 5, 7, 9, The magician then said the remaining number, what is that number.
一位魔术师让观众写下一个六位数a，并将a的各位数字相加得b，他让观众说出a-b中的5个数字，观众报出1、3、5、7、9，魔术师便说出余下的那个数字，那个数字是多少．
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