a)31/80+(3/16+39/80) б)2/45+(3/45+7/9) в)(3/7+5/14)+1/14

a) 31/80+(3/16+39/80)= 31/80+((15+39)/80)=31/80+54/80=85/80=17/16

b) 2/45+(3/45+7/9)=2/45+(3/45+35/45)=2/45+38/45=40/45=8/9

c)(3/7+5/14)+1/14=(6/14+5/14)+1/14=11/14+1/14=12/14=6/7