Since you know 6 plants use 2 bags, you can put it into a ratio of bags/plants, which would be 2/6. you also know that 15 plants use x amount of bags, so to find the variable x you can make the two ratios equal.
2/6=x/15 Here you can cross multiply to get 6x=2*15 which is 6x=30. divide out the 6 to get x by itself to find that x=5.
So Jenny uses 5 bags for 15 plants
