А) 31/80 + (3/16 + 39/80) = 31/80 + 39/80 + 3/16 = 70/80 + 5/80 = 75/80 = 15/16; б) 2/45 + (3/45 + 7/9) = 2/45 + 3/45 + 7/9 = 5/45 + 35/45 = 40/45 = 8/9; в) (3/7 + 5/14) + 1/14 = 3/7 + 5/14 + 1/14 = 6/14 + 6/14 = 12/14 = 6/7; г) 7/15 + (2/15 + 1/5) = 7/15 + 2/15 + 1/5 = 9/15 + 3/15 = 12/15 = 4/5; д) 3/16 + (1/16 + 5/8) = 3/16 + 1/16 + 5/8 = 4/16 + 10/16 = 14/16 = 7/8; е) (1/13 + 1/14) + 2/13 = 1/13 + 1/14 + 2/13 = 3/13 + 1/14 = 42/182 + 13/182 = 55/182.