Maths Olympiad Prep

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Problem 917

AMC 12 late, AIME early
Combinatorics Difficulty 4.0 Prove it Harvard-MIT Mathematics Tournament · United States

The product of the digits of a 5-digit number is 180180. How many such numbers exist?

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

Solution:
Let the digits be a,b,c,d,ea, b, c, d, e. Then abcde=180=22⋅32⋅5a b c d e = 180 = 2^{2} \cdot 3^{2} \cdot 5. We observe that there are 6 ways to factor 180180 into digits a,b,c,d,ea, b, c, d, e (ignoring differences in ordering):
180=1⋅1⋅4⋅5⋅9=1⋅1⋅5⋅6⋅6=1⋅2⋅2⋅5⋅9=1⋅2⋅3⋅5⋅6=1⋅3⋅3⋅4⋅5=2⋅2⋅3⋅3⋅5. 180 = 1 \cdot 1 \cdot 4 \cdot 5 \cdot 9 = 1 \cdot 1 \cdot 5 \cdot 6 \cdot 6 = 1 \cdot 2 \cdot 2 \cdot 5 \cdot 9 = 1 \cdot 2 \cdot 3 \cdot 5 \cdot 6 = 1 \cdot 3 \cdot 3 \cdot 4 \cdot 5 = 2 \cdot 2 \cdot 3 \cdot 3 \cdot 5.
There are (respectively) 60,30,60,120,6060, 30, 60, 120, 60, and 3030 permutations of these breakdowns, for a total of 360360 numbers.

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