In a bag there are only red counters, blue counters, green counters and yellow counters.
A counter is taken at random from the bag.
The table shows the probabilities that the counter will be green or will be yellow.
| Colour | Red | Blue | Green | Yellow |
|---|---|---|---|---|
| Probability | 0.35 | 0.20 |
The probability that the counter will be red is twice the probability that the counter will be blue.
There are 21 green counters in the bag.
Work out the number of red counters in the bag.
