13 A bag contains 1 red disc, 2 blue discs and 3 green discs.
Xanthe chooses a disc at random from the bag. She notes its colour and replaces it. Then Xanthe chooses another disc at random from the bag and notes its colour.
(a) Complete the probability tree diagram showing all the probabilities.
(3)
(b) Calculate the probability that both discs are the same colour. (3)
(c) Calculate the probability that neither disc is red. (2)
Mark scheme (a)
Scheme
Marks
\(\frac{1}{3}\) & \(\frac{1}{2}\) oe
B1
correct structure
B1
all correct
B1
(3)
Notes
B1: correctly placed once
B1: just branches
B1: including probabilities and labels
Mark scheme (b)
Scheme
Marks
\((\frac{1}{6})^2\) or \((\frac{1}{3})^2\) or \((\frac{1}{2})^2\) oe
