Intermediate June 2018 Unit 1 Q4
4. A bag contains a number of different coloured balls.
A ball is selected at random from the bag.
The probability of selecting a blue ball is 0·3.
(a) Why is the following statement incorrect?
Explain your answer clearly. [1]
Explain your answer clearly. [1]
‘More than half the balls in the bag are blue.’
(b) What is the probability that a ball selected at random from the bag is not blue? [1]
(c) There are 50 balls in the bag.
How many of them are blue? [2]
How many of them are blue? [2]
| Scheme | Marks | Comments |
|---|---|---|
| Statement indicating that 0·3 is less than 0·5. OR Statement indicating that probability of selecting a blue ball should be greater than 0·5. OR Statement that refers to a proportion of the balls e.g. ‘(Only) 30% (of the balls) are blue’, ‘(Only) 3/10(ths) (of the balls) are blue’. | E1 | E0 for e.g. 'Fewer than half the balls are blue'. ‘Should be higher’, ‘Would be above 0·3’. Allow correct interpretation of 0·3 e.g. ‘(Only) 30 out of 100 are blue’, ‘(Only) 15 out of 50 are blue’. Accept any indication for 0·5, e.g. ‘half’, ‘½’. |
Corrected by Westie's Workshop
- The board's mark scheme printed “B0 for e.g. 'Fewer than half the balls are blue'”. Changed to “E0 for e.g. 'Fewer than half the balls are blue'” because this part is marked with an explanation mark (E1), so no marks is written E0, not B0.
Original: see the board's mark scheme PDF (link above).
| Scheme | Marks | Comments |
|---|---|---|
| 0·7 or equivalent. | B1 | B0 for incorrect notation e.g. ‘7 out of 10’. |
| Scheme | Marks | Comments |
|---|---|---|
| 0·3 × 50 | M1 | |
| 15 | A1 | If no marks gained, allow SC1 for sight of 15 e.g. ‘15/50’, 15 : 35. |