Question 857455
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Hi,
Let x represent the number of white balls added    
Total of (36 + x) balls: 16 red, 20 blue and x white
P(red ball) = {{{16/(36+x) = 2/5}}} | Cross Multiplying to solve
                  80 = 72 + 2x
                   8 = 2x
                   x = 4, the number of white balls added
CHECKING our answer***
  16/40 = 2/5
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