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Rain or Shine
Predict future weather using the probability that tomorrow is wet given today is wet and the probability that tomorrow is wet given that today is dry.
Knock-out
Before a knockout tournament with 2^n players I pick two players. What is the probability that they have to play against each other at some point in the tournament?
Squash
If the score is 8-8 do I have more chance of winning if the winner is the first to reach 9 points or the first to reach 10 points?
Age 16 to 18
Challenge Level
A bag contains an assortment of red and blue balls. Two balls are drawn at random.
You are told that the probability of drawing two red balls is 5 times the probability of drawing two blue balls. Furthermore, the probability of drawing one ball of each colour is 6 times the probability of drawing two blue balls.
How many blue and how many red balls are in the bag?
NRICH is part of the family of activities in the Millennium Mathematics Project.