In order to select the new board members. the french club held an election 9 out of 18 club in the election voted in the election . what percentage of the members voted

 "50 %" .

  Note:  "9 out of 18" = 9/18 = (9÷9)/(18÷9) = 1/2 .

Also, note that "9" is ONE-HALF of "18".   {"9 * 2 = 18".}.

1/2 = (1*50) / (2*50) = 50/100 = 50% .

one-half = 50% 

50 is "one--half" of 100

1÷2 = 0.5 .

50/100 = 50 ÷ 100 = 0.50 = 0.5 .

50/100 = 50% .

9/18 = 9÷18 = 0.5 .