JAMB - Mathematics (2004 - No. 15)
Some white balls put in a basket containing twelve red balls and sixteen black balls. If the probability of picking a white ball from the baskets is \(\frac{3}{7}\), how many white balls were introduced?
12
21
28
32
Explanation
Number of white balls = x
Number of red balls = 12
Number of black balls = 16
Total number of balls = 28 + x
P(white balls) = 3/7
But P(white balls) \(= \frac{x}{28+x}\\
= \frac{3}{7} = \frac{x}{28+x}\\
3(28 + x) = 7x\\
84 + 3x = 7x\\
7x - 3x = 84\\
4x = 84\\
x = 21\)
Number of red balls = 12
Number of black balls = 16
Total number of balls = 28 + x
P(white balls) = 3/7
But P(white balls) \(= \frac{x}{28+x}\\
= \frac{3}{7} = \frac{x}{28+x}\\
3(28 + x) = 7x\\
84 + 3x = 7x\\
7x - 3x = 84\\
4x = 84\\
x = 21\)
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