а) 4\45 - 1\30 = (430 - 145) / (45*30) = (120 - 45) / 1350 = 75 / 1350 = 1/18

б) 8\25 + 17\20 = (820 + 1725) / (25*20) = (160 + 425) / 500 = 585 / 500 = 117/100