Maths Olympiad Prep

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Number theory Difficulty 4.1 AIME Find the answer United States

Problem:

How many ordered triples of positive integers (a,b,c)(a, b, c) are there for which a4b2c=54000a^{4} b^{2} c = 54000?

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

Solution

Solution:

We note that 54000=24×33×5354000 = 2^{4} \times 3^{3} \times 5^{3}. Hence, we must have a=2a13a25a3a = 2^{a_{1}} 3^{a_{2}} 5^{a_{3}}, b=2b13b25b3b = 2^{b_{1}} 3^{b_{2}} 5^{b_{3}}, c=2c13c25c3c = 2^{c_{1}} 3^{c_{2}} 5^{c_{3}}. We look at each prime factor individually:

- 4a1+2b1+c1=44 a_{1} + 2 b_{1} + c_{1} = 4 gives 4 solutions: (1,0,0),(0,2,0),(0,1,2),(0,0,4)(1,0,0), (0,2,0), (0,1,2), (0,0,4)
- 4a2+2b2+c2=34 a_{2} + 2 b_{2} + c_{2} = 3 and 4a3+2b3+c3=34 a_{3} + 2 b_{3} + c_{3} = 3 each give 2 solutions: (0,1,1),(0,1,3)(0,1,1), (0,1,3).

Hence, we have a total of 4×2×2=164 \times 2 \times 2 = 16 solutions.

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