When A, B and C are deployed for a task , A and B together do 70% of the work and B and C together do 50% of the work. who is most efficient?

A + B= 70%   

B + C =50%   

  $\because (A+B)+(B+C)-(A+B+C)= B$

=> B= 20%     A= 50%        and   C=30%

Hence A is most efficient