(5/21 + 2/15) + 2/21 = 5/21 + 2/15 + 2/21 = 5/21 + 2/21 + 2/15 = 7/21 + 2/15 = 1/3 + 2/15 = 5/15 + 2/15 = 7/15
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(2/35 + 3/28) + 1/28 = 2/35 + 3/28 + 1/28 = 2/35 + 4/28 = 2/35 + 1/7 = 2/35 + 5/35 = 7/35 = 1/5
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(7/30 + 8/45) + (2/45 + 1/30) = 7/30 + 8/45 + 2/45 + 1/30 = 7/30 + 1/30 + 8/45 + 2/45 = 8/30 + 10/45 = 4/15 + 2/9 = 12/45 + 10/45 = 22/45
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(5/24 + 11/60) + (13/60 + 1/24) = 5/24 + 11/60 + 13/60 + 1/24 = 5/24 + 1/24 + 11/60 + 13/60 = 6/24 + 24/60 = 1/4 + 2/5 = 5/20 + 8/20 = 13/20