Maths Olympiad Prep

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Number theory Difficulty 5.4 AIME, harder Find the answer

Find all the three digit numbers abc\overline{a b c} such that

abc=abc(a+b+c) \overline{a b c}=a b c(a+b+c)

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We will show that the only solutions are 135 and 144.

We have a>0,b>0,c>0a > 0, b > 0, c > 0 and

9(11a+b)=(a+b+c)(abc1) 9(11a + b) = (a + b + c)(abc - 1)

- If a+b+c0(mod3)a + b + c \equiv 0 \pmod{3} and abc10(mod3)abc - 1 \equiv 0 \pmod{3}, then abc1(mod3)a \equiv b \equiv c \equiv 1 \pmod{3} and 11a+b0(mod3)11a + b \equiv 0 \pmod{3}. It follows now that

a+b+c0(mod9) or abc10(mod9) a + b + c \equiv 0 \pmod{9} \text{ or } abc - 1 \equiv 0 \pmod{9}

- If abc10(mod9)abc - 1 \equiv 0 \pmod{9}

we have 11a+b=(a+b+c)k11a + b = (a + b + c)k, where kk is an integer

and it is easy to see that we must have 1919.

Now we will deal with the case when a+b+c0(mod9)a + b + c \equiv 0 \pmod{9} or a+b+c=9la + b + c = 9l, where ll is an integer.

- If l2l \geq 2 we have a+b+c18,max{a,b,c}6a + b + c \geq 18, \max\{a, b, c\} \geq 6 and it is easy to see that abc72abc \geq 72 and abc(a+b+c)>1000abc(a + b + c) > 1000, so the case l2l \geq 2 is impossible.
- If l=1l = 1 we have

11a+b=abc1 or 11a+b+1=abc(a+b+c3)3=27 11a + b = abc - 1 \text{ or } 11a + b + 1 = abc \leq \left(\frac{a + b + c}{3}\right)^3 = 27

So we have only two cases: a=1a = 1 or a=2a = 2.

- If a=1a = 1, we have b+c=8b + c = 8 and 11+b=bc111 + b = bc - 1 or b+(c1)=7b + (c - 1) = 7 and b(c1)=12b(c - 1) = 12 and the solutions are (a,b,c)=(1,3,5)(a, b, c) = (1, 3, 5) and (a,b,c)=(1,4,4)(a, b, c) = (1, 4, 4), and the answer is 135 and 144.
- If a=2a = 2 we have b(2c1)=23b(2c - 1) = 23 and there is no solution for the problem.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.