Answer:
A) The probability of picking a yellow ball from the bag is 26.66%. 
B) The probability of picking a blue ball from the bag is 40%. 
C) The probability of picking a yellow ball and then a blue ball is 11.42%.
Explanation:
Since a bag contains 4 yellow balls, 5 red balls, and 6 blue balls, to determine what is the probability of picking a yellow ball from the bag, what is the probability of picking a blue ball from the bag and what is the probability of picking a yellow ball and then a blue ball after replacement, the following calculations must be performed: 
 
A) 
4 + 5 + 6 = 100 
4 = X 
15 = 100 
4 = X 
4 x 100/15 = X 
400/15 = X 
26.66 = X 
 
Therefore, the probability of picking a yellow ball from the bag is 26.66%. 
 
B) 
15 = 100 
6 = X 
6 x 100/15 = X 
600/15 = X 
40 = X 
 
Therefore, the probability of picking a blue ball from the bag is 40%. 
 
C) 
14 = 100 
6 = X 
6 x 100/14 = X 
600/14 = X 
42.85 = X 
 
0.2666 x 0.4285 = X 
0.1142 = X 
 
Therefore, the probability of picking a yellow ball and then a blue ball is 11.42%.