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Answer :
Total number of black balls = x
Total number of balls = 12
\( P(E) \ = \ \frac{Number \ of \ favourable \ outcomes}{Total \ number \ of \ outcomes} \)
P (getting black balls) \(= \ \frac{x}{12} \) ...... (i)
Now, when 6 more black balls are added,
Total balls become = 18
\(\therefore \) Total number of black balls = x + 6
Now, P (getting black balls) \( = \ \frac{x \ + \ 6}{18} \) ....... (ii)
Solving equation (i) and (ii)
x = 3