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4 - 1 9/10 = 3 10/10 - 1 9/10 = 2 1/10,

9/25 : 2 1/10 = 9/25 : 21/10 = 9/25 * 10/21 = 6/35,

3/10 - 6/35 + 4/7 = 21/70 - 12/70 + 40/70 = 49/70 = 7/10

ответ: 7/10 или 0,7