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Combinatorics Difficulty 3.2 AMC 10/12 Find the answer

Let nn be a positive integer. If the equation 2x+2y+z=n2x+2y+z=n has 28 solutions in positive integers xx, yy, and zz, then nn must be either
(A) 14 or 15 (B) 15 or 16 (C) 16 or 17 (D) 17 or 18 (E) 18 or 19\mathrm{(A)}\ 14 \text{ or } 15\ \mathrm{(B)}\ 15 \text{ or } 16\ \mathrm{(C)}\ 16 \text{ or } 17\ \mathrm{(D)}\ 17 \text{ or } 18\ \mathrm{(E)}\ 18 \text{ or } 19

Multiple choice: answer with the letter of the option you want.

Solution

This is equivalent to finding the powers of kk with coefficient 2828 in the expansion of (k2+k4+k6+k8+...)2(k+k2+k3+k4+...)(k^2+k^4+k^6+k^8+...)^2(k+k^2+k^3+k^4+...).
But this is (k21k2)2k1k = k5(11k)3(11+k)2 = k5(1+k)(11k2)3\left(\frac{k^2}{1-k^2}\right)^2\cdot\frac{k}{1-k}\ =\ k^5\cdot\left(\frac{1}{1-k}\right)^3\cdot\left(\frac{1}{1+k}\right)^2\ =\ k^5\cdot(1+k)\cdot\left(\frac{1}{1-k^2}\right)^3
=k5(1+k)((22)+(32)k2+(42)k4+(52)k6+...)=k^5(1+k)(\tbinom{2}{2}+\tbinom{3}{2}k^2+\tbinom{4}{2}k^4+\tbinom{5}{2}k^6+...), the last part having general term (j2)k2j4\tbinom{j}{2}k^{2j-4}
from which it is easy to see that, since (82)=28\tbinom{8}{2}=28, the last part contains the term 28k1228k^{12} and the whole result 28k17+28k1828k^{17}+28k^{18}. So the answer is (D)\mathrm{(D)}.

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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.