1990 AHSME Problems/Problem 29
Contents
Problem
A subset of the integers 
has the property that none of its members is 3 times another. What is the largest number of members such a subset can have?
Solution 1
Notice that inclusion of the integers between 
to 
inclusive is allowed as long as no integer between 
and 
inclusive is within the set. This provides a total of 
= 67 solutions.
Further analyzing the remaining integers between 
and 
, we notice that we can include all the numbers except 
(as including would force us to remove both 
and ) to obtain the maximum number of solutions.
Thus, 
, yielding our answer, 
Solution 2
Write down in a column the elements 
which are indivisible by three, and then follow each one by 
![\[\begin{array}{ccccc}1&3&9&27&81\\2&6&18&54\\4&12&36\\5&15&45\\7&21&63\\8&24&72\\10&30&90\\11&33&99\\13&39\\\vdots&\vdots\end{array}\]](http://latex.artofproblemsolving.com/1/2/6/1261b43b33557578b2ffd319b00ef4d01d697fb4.png)
We can take at most elements from the first row, and at most 
elements from each of the next seven rows. After that we can take only from any following row. Thus the answer is 
the number of integers between 
and inclusive which are indivisible by three.
There are 
multiples of three in that range, so there are 
non-multiples, and 
, which is
See also
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