Problem:
Compute the number of ways to pick a 3-element subset of
such that the product of the 3 numbers in the subset has no digits besides 0 and 1 when written in base 10.
Solution
Solution:
Given a subset , we can directly expand the product of its elements:
In order for all digits to be 0 or 1, all 7 numbers should be distinct and nonzero, with the latter being guaranteed.
Without loss of generality, we can assume . Then,
and
so the only two numbers that could be the same are and .
There are 9 triples where and , namely , , and . The remaining triples all work, so the answer is .
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