I found that Joe has 28 red pepper plants and 8 green pepper plants.
to find this, I first set up a proportion:
7/2 = (20+x)/x
then you cross multiply giving you:
7x = 40 + 2x
Subtract 2x from both sides:
5x = 40
solve for x:
x = 8 (which is the amount of green pepper plants)
to find how many red pepper plants he has, plug in x for (20 + x) giving you 28 red pepper plants
Hope I helped :)