There are 5 girls, 6 boys and some adults in a classroom. The probability that a girl is chosen is 1/3. What is the probability of an adult being chosen?

This is a question taken straight out of an Edexcel exam paper. I think it's a good test of GCSE level probability and algebra. We don't know how many adults there are to begin with so let's say n = number of adults.So the total number of people to choose from is 5 + 6 + n which is the same as 11 + n.The probability of picking a girl from this number of people is 5/(11+n) (You can confirm this with a tree diagram if it's easier to visualise).Since the question tells us this value is equal to 1/3, we must solve: 5/(11+n) = 1/3 to find n.Cross multiplying we get 11 + n = 15 which then means n = 4 and the total number of people is 15.The probability then of picking an adult is 4/15.