There are 15 tennis balls in a box, of which 9 have not previously been used. three of the balls are randomly chosen, played with, and then returned to the box. later, another 3 balls are randomly chosen from the box. find the probability that none of these balls has ever been used

Total tennis balls = 15 balls
Previously not used balls = 9 balls
Previously used balls = 15 - 9 = 6 balls

Probability of getting first 3 balls that are not used previously =  ×   ×  

Now remaining total balls = 15 - 3 = 12
Balls that are not previously used = 9 - 3 = 6

Probability of getting next 3 balls that are not used previously =  ×   ×  

Probability =  ×   ×   ×  ×   ×  

= 0.089