A box contains 4 black and 5 red balls and another box contains 5 black and 4 red balls. One ball is to drawn from either of two boxes. What is the probability of drawing a black ball ? Choose the correct option:
A) 1/9
B) 2/9
C) 4/9
D) 1/2
E) 2/3
Explanation :   Probability of choosing first box is 1/2 and second box is 1/2.   ∴ Probability of choosing one balck ball from the first box is 4C1 / 9C1 x 1/2 = 4/9 x 1/2 = 2/9   Probability of choosing one black ball from the second box is = 1/2 x 5C1 / 9C1 = 5/18   ∴ P(E) = 2/9 + 5/18 = 4 + 5/18 = 9/18 = 1/2   Answer : Option 1/2
<result>D</result>
A box contains 4 black and 5 red balls and another box contains 5 black and 4 red balls. One ball is to drawn from either of two boxes. What is the probability of drawing a black ball ?