Higher June 2018 Paper 1 Q16
16 There are only red counters, blue counters and purple counters in a bag.
The ratio of the number of red counters to the number of blue counters is 3 : 17
Sam takes at random a counter from the bag.
The probability that the counter is purple is 0.2
Work out the probability that Sam takes a red counter. (3)
| Answer | Mark | Mark scheme |
|---|---|---|
| 0.12 | P1 | for process to start eg \((1 - 0.2) \div (3 + 17)\) (= 0.04) or \((3 + 17) \div (1 - 0.2)\) oe (= 25) or \((100 - 20) \div (3 + 17)\) (= 4) or \(3 \times 4\) (= 12) and \(17 \times 4\) (= 68) |
| P1 | full process to find the required probability eg \(3 \times \text{``}0.04\text{''}\) or \(\dfrac{3}{20} \times (1 - 0.2)\) oe or \(3 \div \text{``}25\text{''}\) or \(3 \times \text{``}4\text{''} \div 100\) | |
| A1 | oe |
Additional guidance
Just \(1 - 0.2 = 0.8\) is not sufficient for P1
May be seen in a ratio
0.12 using incorrect probability notation gets P2