X and Y are positive two-digit integers such that the tens digit in X is the same as the units digit in Y, and the units digit in X is the same as the tens digit in Y. Which of the following CANNOT be the difference of X and Y ?  Select the correct option
A) 72 B) 45 C) 27 D) 33 E) 9
X and Y are positive two-digit integers such that the tens digit in X is the same as the units digit in Y, and the units digit in X is the same as the tens digit in Y:   X = 10a + b   Y = 10a + b   X - Y = (10a + b) - (10a + b) = 9(a - b). As you can see the difference must be a multiple of 9. Only option D is not a multiple of 9.   Answer: 33.
<result>D</result>
X and Y are positive two-digit integers such that the tens digit in X is the same as the units digit in Y, and the units digit in X is the same as the tens digit in Y. Which of the following CANNOT be the difference of X and Y ?