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1 / 4 - 1 / 5 = 5 / 20 - 4 / 20 = 1 / 20 1 / 4 + 2 / 3 = 3 / 12 + 8 / 12 = 11 / 12 3 / 7 - 1 / 5 = 15 / 35 - 7 / 35 = 8 / 35 3 / 7 + 2 / 3 = 9 / 21 + 14 / 21 = 23 / 21 2 / 9 - 1 / 5 = 10 / 45 - 9 / 45 = 1 / 45 2 / 9 + 2 / 3 = 2 / 9 + 6 / 9 = 7 / 9.

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7

1 / 4 - 1 / 5 = 5 / 20 - 4 / 20 = 1 / 20

1 / 4 + 2 / 3 = 3 / 12 + 8 / 12 = 11 / 12

3 / 7 - 1 / 5 = 15 / 35 - 7 / 35 = 8 / 35

3 / 7 + 2 / 3 = 9 / 21 + 14 / 21 = 23 / 21

2 / 9 - 1 / 5 = 10 / 45 - 9 / 45 = 1 / 45

2 / 9 + 2 / 3 = 2 / 9 + 6 / 9 = 7 / 9.