The ratio for this situation would be 20:6, as this represents 6 red cars for every 20 cars sold. To get the fraction, just add the ratio on each side together, then place the number you want as a fraction on top, then simply if possible. This looks like this:
20+6=26
6/26
Divide the numerator and denominator by 2
3/13
So, the fraction is 3/13