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