1) 3/14 + 1/21= 9/42 + 2/42= 11/42

2) 11/42 : 33/56= (11×56) / (42×33)= 4/9

3) 4/9 × 9/16= (4×9) / (9×16)= 1/4= 0.25

4) 7/9 - 1/6= 14/18 - 3/18= 11/18

5) 11/18 : 55/72= (11×72) / (18×55)= 4/5= 0.8

6) 0.25 : 0.8= 2.5 : 8= 0.3125