15 1/2 - (6 1/4 + 2 3/8) = 15.5 - (6.25 + 2.375) = 15.5 - 8.625 = 6.875
а) 2/15 + (1/15 + 1/3) = 2/15 + 4/15 = 6/15 = 2/5
б) 1/20 + 2/15 + 3/20 + 1/15 = 1/20 + 4/30 + 3/20 + 2/30 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20 + 2/15 + 3/20 = 1/20 + 4/30 + 3/20 = 1/20
в) 3/4 + 2/3 - 17/12 = 9/12 + 8/12 - 17/12 = (9 + 8 - 17) / 12 = 0 / 12 = 0