In a jar there are only white marbles, black marbles, orange marbles and purple marbles.
A marble is taken at random from the jar.
The table shows the probabilities that the marble will be orange or will be purple.
| Colour | White | Black | Orange | Purple |
|---|---|---|---|---|
| Probability | 0.25 | 0.15 |
The probability that the marble will be black is three times the probability that the marble will be white.
There are 20 orange marbles in the jar.
Work out the number of black marbles in the jar.
