a)C1  (6/11+5/11)*3/7=1*3/7=3/7         C2   (6/11=5/11)*3/7=6/11*3/7+5/11*3/7=18/17+16/17=3/7

b) C1 3/5*7/9-3/5*2/9=7/15-2/15=1/3         C2 3/5*7/9-3/5*2/9=3/5*(7/9-2/9)=3/5*5/9=1/3

c) C1(6/7-4/7):2/5==2/7:2/5=5/7          C2    (6/7-4/7):2/5=6/8:2/5-4/7:2/5=15/7-10/7=5/7

d) C1 8/15:2/11+7/15:2/11=44/15+77/30=11/2          C2 8/15:2/11+7/15:2/11=(8/15+7/15):2/11=1:2/11=11/2