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 28 green pens in the box.
Work out the number of black pens in the box.
