Maths Olympiad Prep

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Number theory Difficulty 5.2 AIME, harder Prove it United States

Problem:

How many solutions does 26=26 = twelve ++ eleven ++ two ++ one have over the positive integers? (Each letter is a variable, and letters in the same word are multiplied.)

Solution

Solution:

Factoring, we get 26=26 = twelve ++ eleven ++ two ++ one =(elve+o)(tw+ne)= (elve + o)(tw + ne).

Both factors are at least 1+1=21 + 1 = 2, so there are two cases: either elve+o=2elve + o = 2 and tw+ne=13tw + ne = 13, or elve+o=13elve + o = 13 and tw+ne=2tw + ne = 2.

In the first case, elve=o=1elve = o = 1, so e=l=v=o=1e = l = v = o = 1. We are then left with tw+ne=13tw + ne = 13.

In the second case, tw=ne=1tw = ne = 1, so t=w=e=n=1t = w = e = n = 1. We are then left with lv+o=13lv + o = 13.

Each case has the same number of solutions as ab+c=13ab + c = 13. For each aa, there are 12a\left\lfloor \frac{12}{a} \right\rfloor multiples of aa under 1313. Thus the number of solutions to this equation is
a=11212a=12+6+4+3+2+2+1+1+1+1+1+1=35 \sum_{a=1}^{12} \left\lfloor \frac{12}{a} \right\rfloor = 12 + 6 + 4 + 3 + 2 + 2 + 1 + 1 + 1 + 1 + 1 + 1 = 35
so the number of solutions to the original problem is 235=702 \cdot 35 = 70.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.