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.
Practise OCR GCSE Maths Probability with exam-style questions for Foundation and Higher tier. 100 questions, matched to the OCR GCSE Maths (J560) specification and written in the written papers style. Every question includes a full worked solution and mark scheme, so you can see where marks are awarded rather than just whether you got the answer right.