A bag contains 2 yellow marbles, 3 green marbles and 7 white marbles, and one marble is taken at random. What is the probability that it is yellow or green? Give the probability as a fraction out of the total number of marbles.

Explication

Two yellow and three green marbles make 5 favourable marbles among the 12 in the bag, so the draw succeeds in 5 cases out of 12. Answering 5/24 comes from adding the bottom numbers of 2/12 and 3/12 as well as the top ones, but the bag does not gain marbles when two colours are considered together, so the total stays at 12.