Olympiad Maths Prep

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Number theory Difficulty 5.0 AIME, harder Prove it Romania

Find all three-digit numbers which decrease 13 times when the tens' digit is suppressed.

Solution

Let abc\overline{abc} be such a number. The condition is abc=13ac\overline{abc} = 13 \cdot \overline{ac}, that is 100a+10b+c=130a+13c100a + 10b + c = 130a + 13c, same as 10b=30a+12c10b = 30a + 12c. We notice that 55 divides both 10b10b and 30a30a, so 12c12c is divisible by 55, hence c{0,5}c \in \{0, 5\}.

If c=0c = 0, then b=3ab = 3a. Since aa is a non-null digit, (a=1,b=3)(a = 1, b = 3), (a=2,b=6)(a = 2, b = 6) and (a=3,b=9)(a = 3, b = 9), which lead to the solutions 130130, 260260 and 390390.

If c=5c = 5, then b=3a+6b = 3a + 6. Since aa is a non-null digit, the only possibility is (a=1,b=9)(a = 1, b = 9), which leads to the solution 195195.

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