<PRE><B>Question 942298</B>
  
Given:
3 red, 4 blue and 5 black balls.
  
Find:
minimum number of balls required to make sure (probability 100%) that there will be at least one ball of each colour.
  
Solution:
The worst case is drawing 5 black balls, followed by 4 blue balls, and we still do not have one of each colour yet.  So it takes a 10th ball to make sure (100% probability) that there is at least one ball of each colour.
  
Answer: It will take at least 10 balls to be sure of getting one of each colour.
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