Question on: WAEC Mathematics - 2009
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
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Correct Option: 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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