We find the common factor and reduce the fraction:
10/15 = (10/5) / (15/5) = 2/3
15/20 = (15/5) / (20/5) = 3/4
2/6 = (2/2) / (6/2) = 1/3
2/8 = 1/4
3/12 = 1/4
3/9 = 1/3
4/6 = 2/3
5/15 = 1/3
5/20 = 1/4
6/8 = 3/4
6/9 = 2/3
9/12 = 3/4
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