Hi!
It can be many other numbers, but the best are that are in some connection with 18, but that are not bigger than 18, like this:
1/3 = 1*2/3*2 = 2/6
1/3 = 1*3/3*3 = 3/9
1/3 = 1*6/3*6 = 6/18
Hope this helps!
6/18
3/9
12/36
5/15
4/12
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