Maths Olympiad Prep

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, 2008

Algebra Difficulty 4.7 AIME Prove it Slovenia

Find the smallest three-digit integer with the property that its triple has only even digits.

Solution

Denote the three-digit number by abc\overline{abc}. Its triple is equal to
3abc=(3a)100+(3b)10+3c. 3 \cdot \overline{abc} = (3a) \cdot 100 + (3b) \cdot 10 + 3c.
Obviously, aa has to be at least 1. If a=1a = 1 and we want the digit at the hundreds in 3abc3 \cdot \overline{abc} to be even, we require 3b10+3c1003b \cdot 10 + 3c \ge 100, which implies 10b+c1003=33+1310b + c \ge \frac{100}{3} = 33 + \frac{1}{3}. The least possible number that satisfies this inequality is 34. The solution is 134 and its triple is 402.

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