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counting and probability senior

Problem

A rising number, such as , is a positive integer each digit of which is larger than each of the digits to its left. There are five-digit rising numbers. When these numbers are arranged from smallest to largest, the number in the list does not contain the digit
(A)
(B)
(C)
(D)
Solution
The list starts with . There are four-digit rising numbers that do not begin with , and thus also five digit rising numbers that do begin with that are formed by simply putting a before the four digit number. Thus, the number is . There are three-digit rising numbers that do not begin with a or , and thus five digit rising numbers that begin with a . Thus, the number is . Counting up, is the number, which does not contain the digit .
Final answer
B