You may also like

problem icon

Gambling at Monte Carlo

A man went to Monte Carlo to try and make his fortune. Whilst he was there he had an opportunity to bet on the outcome of rolling dice. He was offered the same odds for each of the following outcomes: At least 1 six with 6 dice. At least 2 sixes with 12 dice. At least 3 sixes with 18 dice.

problem icon

Balls and Bags

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

problem icon

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?

Weekly Problem 45 - 2012

Stage: 4 Challenge Level: Challenge Level:1


There are $10$ girls in a mixed class.

If two pupils from the class are selected at random, then the probability that both are girls is $0.15$.

How many boys are in the class?




If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.






This problem is taken from the UKMT Mathematical Challenges.

View the previous week's solution
View the current weekly problem