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?

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

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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?

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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?