1 1/8 + 2/4 = (8x1+1)/8 + 2/4 = 9/8 + 2/4 = (mmc=8) = (9+4)/8 = 13/8

2 1/3 + 1 1/2 = (3x2+1)/3 + (2x1+1)/2 = 7/3 + 3/2 = (mmc=6) = (14 + 9)/6 = 23/6

2  1/4 - 1 1/8 = (4x2+1)/4 - (8x1+1)/8 = 9/4 - 9/8  = (mmc = 8) = (18 - 9)/8 = 9/8

2  1/6  - 1  1/2 = (6x2+1)/6 - (2x1+1)/2 = 13/6 - 3/2 = (mmc = 6) = (13 - 9)/6 = 4/6 = 2/3

Espero ter ajudado.