In a box there are only red pens, blue pens, black pens and green pens. A pen is taken at random from the box. The table shows the probabilities that the pen will be red or will be green.
| Colour | Red | Blue | Black | Green |
|---|---|---|---|---|
| Probability | 0.42 | 0.14 |
The probability that the pen will be black is three times the probability that the pen will be blue. There are k k\,k green pens in the box.
Work out the number of black pens in the box in terms of kkk.
324 exam-style questions on AQA GCSE Maths Probability. Each one has a worked solution and a mark scheme showing where the marks go.