Question

The probability of drawing a black marble from a bag is 1/2, and the probability of drawing a white marble from the same bag is 1/4. What is the probability of drawing a white marble, replacing it, and then drawing a black marble?
1/8
1/3
3/4

Answer 1

W = event where white marble is drawn 
B = event where black marble is drawn 

The two events W and B are independent. This is because the marble taken out on the first draw is put back into the bag. Since W and B are independent, we can use this rule 

P(W and B) = P(w)*P(B) 


Use the rule to get 

P(W and B) = P(W)*P(B)
P(W and B) = (1/2)*(1/4)
P(W and B) = (1*1)/(2*4)

P(W and B) = 1/8 

1/8 is equivalent to 0.125. Use long division or a calculator 

0.125 = 12.5% 

Answer 2

Answer is 1/8. this is because 1/2*1/4 equals 1/8.