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?
A
12
B
21
C
28
D
32
Verified Correct Answer: B.
Step-by-Step Mathematical Solution
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
Source: Myschool Past Questions Archive (Record #8189)