Some white balls put in a basket containing twelve red balls and sixteen black balls. If the probability of picking a white balls from the baskets is 3/7, how many white balls were introduced?
A. 12 B. 21 C. 28 D. 32
Correct Answer: B
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) \(= \frac{x}{28+x}\\ = \frac{3}{7} = \frac{x}{28+x}\\ 3(28 + x) = 7x\\ 84 + 3x = 7x\\ 7x - 3x = 84\\ 4x = 84\\ x = 21\)
