а)4,2/25,2 = (21/5) / (126/5) = 21/126 = 1/6
б)2_2/5 / 1_1/5 = (4/5) / (1/15) = 12
в) ((4_2/7)×(1_3/4) - 3,36) / (0,8 + 1,5) = (2/7 × 3 - 84/25) / 2,3 = (6/7 - 84/25) / (23/10) = (-438/175) / (23/10) = - ((438/175) / (23/10)) = - (876/805)
