Mean changing with changing frequencies and class intervals

Question

Let $A$ denotes weekly wages and $B$ denotes Number of workers.

\begin{array} {|r|r|}\hline A & 50-55 & 55-60 & 60-65 & 65-70 & 70-75 & 75-80 & 80-85 & 85-90 \\ \hline B & 5 & 20 & 10 & 10 & 9 & 6 & 12 & 8 \\ \hline \end{array}

1. Find mean
2. New mean if wages of each worker are increased by $20 %$
3. New mean if number of workers in category doubled and wages were decreased by $ 10 %$

Also could somebody please tell me how mean changes with changing frequencies and changing class intervals?