Q:
      
      
         
            
In a box, there are 9 blue, 6 white and some black stones. A stone is randomly selected and the probability that the stone is black is ¼. Find the total number of stones in the box? 
         
       
      
      
      
          
      
      
          Answer & Explanation
         Answer: C) 20         
         
Explanation: We know that, Total probability = 1
Given probability of black stones = 1/4
=> Probability of blue and white stones = 1 - 1/4 = 3/4
But, given blue + white stones =  9 + 6 = 15
Hence,
3/4 ----- 15
 1   -----  ?
=> 15 x 4/3 = 20.
 
Hence, total number of stones in the box = 20.
       
      
      
      
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