Q:
      
      
         
            
A bag contains 4 red and 3 black balls. A second bag contains 2 red and 4 black balls. One bag is selected at random. From the selected bag, one ball is drawn. Find the probability that the ball drawn is red.
         
       
      
      
      
          
      
      
          Answer & Explanation
         Answer: B) 19/42         
         
Explanation: A red ball can be drawn in two mutually exclusive ways
 (i) Selecting bag I and then drawing a red ball from it.
 
(ii) Selecting bag II and then drawing a red ball from it.
 
Let E1, E2 and A denote the events defined as follows:
E1 = selecting bag I,
E2 = selecting bag II
A = drawing a red ball
Since one of the two bags is selected randomly, therefore 
P(E1) = 1/2  and  P(E2) = 1/2
Now,  = Probability of drawing a red ball when the first bag has been selected = 4/7
    = Probability of drawing a red ball when the second bag has been selected = 2/6
 Using the law of total probability, we have 
 P(red ball) = P(A) =  
 
                          = 
       
      
      
      
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