Ответ:

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1) 5/7 + 1/4 = (5/7)*4 + (1/4)*7 = 20/28 = 7/28 = 13/28

2) 7/15 - 3/10 = (7/15)*2 + (3/10)*3 = 14/30 - 9/30 = 5/30 = 1/6

3)11/18 - 4/9 = (11/18)*1 - (4/9)*2 = 11/18 - 8/18 = 3/18 = 1/6

4)7/30 + 9/20 = (7/30)*2 + (9/20)*3 = 14/60 + 27/60 = 41/60

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