A box contains black, white and red identical balls. The probability of picking a black ball at random from the box is \(\frac{3}{10}\) and the probability of picking a white ball at random is \(\frac{2}{5}\). If there are 30 balls in the box, how many of them are red?
A. 3 B. 7 C. 9 D. 12
Correct Answer: C
Explanation
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
