19. For her lunch, Gina sometimes eats a salad and sometimes drinks a coffee. The probability she eats a salad for lunch on a given day is 0·4. The probability she drinks a coffee on a given day is 0·25. Eating a salad for lunch and drinking a coffee are independent.
(a) Complete the following tree diagram. [2]
(b) Calculate the probability that, on a given day, Gina does not eat a salad for lunch but does drink a coffee. [2]
Mark scheme (a)
Scheme
Marks
Comments
Correctly completing the tree diagram 0.6, 0.75, 0.25, 0.75 oe
B2
B1 for a correct pair of branches (total 1)
Mark scheme (b)
Scheme
Marks
Comments
0.6 × 0.25
M1
FT their tree diagram provided their values are between 0 and 1
= 0.15 oe
A1
FT Ignore incorrect conversion of ‘their 0.15’ to another form if the correct answer seen. Do not ignore further calculations.
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