1/4 + 1/3 = (1*3 + 1*4)/12 = (3 + 4)/12 = 7/12
1/3 + 1/9 = (1*3 + 1*1)/9 = (3 + 1)/9 = 4/9
1/2 - 1/3 = (1*3 - 1*2)/6 = (3 - 2)/6 = 1/6
7/8 - 1/2 = (7*1 - 1*4)/8 = (7 - 4)/8 = 3/8
1/2 + 1/4 +5/8 = (1*4 + 1*2 + 5*1)/8 = (4 + 2 + 5)/8 = 11/8 = 1 3/8
5/6 - 1/2 - 1/8 = (5*4 - 1*12 - 1*3)/24 = (20 - 12 - 3)/24 = 5/24