Can you please show me how to answer these discrete math questions, thank you!

6. Consider three sets A, B, C. Prove or disprove that (AB)C=A(BC). 7. Show that for any integer n there are n consecutive composite integers. 8. What is the most appropriate "big-O" expression for the complexity of the following loop (justify your answer): for I:=1 to n2 do for J:=1 to n do k:=j while k>0 do k:=k/2 end; end; end