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: Option 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)