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: Option 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