Magic balls
Probability
Level
3
You have five balls, a white one, a black one and two red ones, out of which you extract two. You know that one of them is red. What is the possibility that the other one is red as well? (Only write the percentage)
Hint: it is a whole number
The answer is 20.
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1 solution
If we think of the two balls that we extract in terms of combinatorics, not individually, we could have 6 combinations: white+black, white+red1, white+ed2, black+red1, black+red2 and red1+red2. Out of these cases, the first one is impossible, as we already know that one of the balls we extracted is red. So there are 5 cases left, out of which only the last one (red ball1+ red ball2) is desired. This means that the possibility is 1/5=20%