Setting the equation up is probably the hardest part of this question. In this case, you're told that 3 out of every 20 trees has been cut down, which is the "ratio"; if a total of 840 trees have been cut down, there should be many more that are left since for every 20 trees, only three are cut down.
Set the equation up as:
3 / 20 = 840 / x
where x = the number of trees left standing.
Now, you just cross multiply and solve:
3 / 20 = 840 / x
3x = (840 x 20)
3x = 16800
x = 16800 / 3
x = 5600
Hope this helps!
Good luck
