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а)

(13 93/98 - 15 17/49) - (13 93/98 - 2 5/7) =
= 13 93/98 - 15 17/49 - 13 93/98 + 2 5/7 = 
= -15 17/49 + 2 5/7 = 
= -14 66/49 + 2 35/49 = 
= - 12 31/49,

б)

6 5/13 * (-5 7/9) - 6 5/13 * 2 8/9 = 
= 6 5/13 * (-5 7/9 - 2 8/9) =
= 6 5/13 * (-7 16/9) =
= 6 5/13 * (-8 7/9) =
= - 83/13 * 79/9 = 
= - 6557/117 =
= - 56 5/117