3
7
9
12
Correct answer is C
Total no of balls = 30
Let x = no. of red balls
Pr(red) = \(\frac{x}{30}\)
Pr(black) = \(\frac{3}{10} = \frac{9}{30}\)
Pr(white) = \(\frac{2}{5} = \frac{12}{30}\)
No. of black balls = 9
No. of white balls = 12
9 = 12 + x = 30
x = 30 - 21
x = 9
No. of red balls = 9