27.
12 3/4-1 1/4-3 3/4+2 1/4=51/4+5/4-15/4+9/4=50/4=12 5/10=12 1/2
5 9/35-2 22/35-1 12/35+10 26/35=184/35-92/35-47/35+376/35=421/35=12 1/35
28.
(3 16/27)-(2 25/36+1 7/36)=97/27+44/27-97/36+43/36=87/36=2 15/36=2 5/12
(3 25/48-1 35/48)+(4 65/72-2 68/72)=(169/48+83/48)+(353/72-212/72)=
= 252/48-141/72=1512/288-564/288=948/288=3 84/288 = 3 7/24