There are some green counters and some red counters in a bag. At the start, 7 of the counters are green, the rest are red.
Jacinta takes at random a counter from the bag. She does not put the counter back in the bag.
Jacinta then takes at random another counter from the bag.
The probability that the first counter Jacinta takes is red and the second is green is 21/80
Work out the number of red counters in the bag at the start.