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Two bags contain different numbers of red and blue marbles. A marble is removed from one of the bags. The marble is blue. What is the probability that it was removed from bag A?

Coin Tossing Games

You and I play a game involving successive throws of a fair coin. Suppose I pick HH and you pick TH. The coin is thrown repeatedly until we see either two heads in a row (I win) or a tail followed by a head (you win). What is the probability that you win?

Thank Your Lucky Stars

Age 14 to 16 Challenge Level:

Start with the simple problem, where you need to flip the coin twice:

Which moves are possible?
Which moves lead you to the target?

List the possibilities.

Move to the harder problem whereyou need to flip the coin four times:

Which moves are possible?
Which moves lead you to the target?

List the possibilities.

Move to the hardest problem...
The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.
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