A "double-single" number is a three-digit number made up of two identical digits followed by a different digit. For example, 553 is a double-single number. How many double-single numbers are there between 100 and 1000 ?
(A) 81
(B) 18
(C) 72
(D) 64
(E) 90
A "double-single" number is of the form $a a b$, where $a$ and $b$ are different digits. There are 9 possibilities for $a$ (since $a$ cannot be 0 ). For each of these possibilities, there are 9 possibilities for $b$ (since $b$ can be any digit from 0 to 9 , provided that it doesn't equal $a$ ). So there are $9 \times 9=81$ possibilities in total.

ANSWER: (A)

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