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) =  | Cross Multiplying to solve

                  80 = 72 + 2x

                   8 = 2x

                   x = 4, the number of white balls added

CHECKING our answer***

  16/40 = 2/5

Wish You the Best in your Studies.

 

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