Maths Olympiad Prep

Track / Stage 3 / 92 of 260 #92 of 1964

Problem 92

AMC 10/12, early questions
Number theory Difficulty 3.4 Multiple choice

The number 20132013 has the property that its units digit is the sum of its other digits, that is 2+0+1=32+0+1=3. How many integers less than 20132013 but greater than 10001000 have this property?

Pick one

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Official solution

We take cases on the thousands digit, which must be either 11 or 22:
If the number is of the form 1bcd,\overline{1bcd}, where b,c,db, c, d are digits, then we must have d=1+b+c.d = 1 + b + c. Since d9,d \le 9, we must have b+c91=8.b + c \le 9 - 1 = 8. By casework on the value of bb, we find that there are 1+2++9=451 + 2 + \dots + 9 = 45 possible pairs (b,c)(b, c), and each pair uniquely determines the value of dd, so we get 4545 numbers with the given property.
If the number is of the form 2bcd,\overline{2bcd}, then it must be one of the numbers 2000,2001,,2012.2000, 2001, \dots, 2012. Checking all these numbers, we find that only 20022002 has the given property.
Therefore, the number of numbers with the property is 45+1=(D)4645 + 1 = \boxed{\textbf{(D)}46} .

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.