In a bag there are only yellow, white, purple and orange marbles. A marble is taken at random from the bag. The table shows the probabilities that the marble will be yellow or will be orange.
| Colour | Yellow | White | Purple | Orange |
|---|---|---|---|---|
| Probability | 0.3 | 0.1 |
The probability that the marble will be white is twice the probability that the marble will be purple. There are 20 orange marbles in the bag.
Work out the number of white marbles in the bag.
