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Gambling at Monte Carlo

A man went to Monte Carlo to try and make his fortune. Is his strategy a winning one?

Marbles and Bags

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?

CD Collection

Age 14 to 16 Short
Challenge Level


First, we need to decide which artist will go first, which will go second, and which will go third.

There are 3 options for the artist that goes first, and then 2 options remaining for the artist that goes second, and then only 1 option left for the artist that goes third. So there are 3$\times$2$\times$1 = 6 ways to decide which artist will go first, which will go second, and which will go third.

There are 2 Beatles CDs, so for each position that they could be in relative to the other CDs, there are 2 options for the Beatles CDs - because they could go either way round.

Similarly, since there are 3 Elvis Presley CDs, for each position that they could be in relative to the other CDs, there are 3 options for the first Elvis Presley CD, 2 options for the second and only 1 option for the third. So there are 3$\times$2$\times$1 = 6 ways to arrange the Elvis Presley CDs.

So altogether, there are 6$\times$2$\times$6 = 72 ways to arrange the CDs.
You can find more short problems, arranged by curriculum topic, in our short problems collection.