例题：AEAS数学逻辑讲解（三）

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　　If xy represents a two-digit number, where x and y are positive single digits, which of the following CANNOT be true?

　　如果XY代表一个两位数，且x和 y是一位数且都是正整数，以下哪个结论不可能是正确的？

　　a.x + y = 6x

　　b.x – y = 9

　　c.x = y

　　d.7x = y

　　Explanation：

　　Step 1

　　x + y = 6x

　　⇒ y = 6x – x

　　⇒ y = 5x

　　The digit y of the number xy is 5 times the digit x if xy = 15.

　　15 is a two digit number of positive digits and hence x + y = 6x is true.

　　Step 2

　　x – y = 9 is possible only if 9 – 0 = 9

　　So, y = 9 and x = 0 and the number would be 09, which is one digit number and hence, x – y = 9 cannot be true.

　　Step 3

　　It is given that x and y are positive.

　　Both digits of the number xy may be equal if the number xy = 22. 22 is a two digit number of positive digits and hence x = y is true.

　　Step 4

　　7x = y

　　The digit y of the number xy is 7 times the digit x, if xy = 17.

　　17 is a two digit number of positive digits and hence 7x = y is true.

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