Question 711148
 % of the people vaccinated =  25%  
   But vaccine has a 90% probability of being effective in preventing a certain disease. 
 % probability not being effective  = 10 %
 % people may get disease  = 25% * 10%  = 2. 5% 
 75 % of the people has not vaccinated 
the probability of getting a certain disease if a person is not vaccinated is 50% 
  % people may get disease  =  75 % * 50 %
                            = 37.5 %
 total % of people may get disease  =  37.5 %+  2.5 %
                                    = 40 %
  so 0ut 0f 100 people , 40 people may get  disease
 Hence  If a person is selected at random, find the probability that he or she will contract the disease  is  40 / 100 
                               =4/10
                                =2/5   or simply 40 %