а) 1/4 + 4/7 + 9/28 = 7/28 + 16/28 + 9/28 = 32/28 = 8/7

б) 1/5 + 3/10 = 2/10 + 3/10 = 5/10 = 1/2

в) 3/20 + 7/30 + 2/40 = 3/20 + 7/30 + 1/20 = 9/60 + 14/60 + 3/60 = 26/60 = 13/30