Answer:
You cross multiply.
Explanation:
Ex.
9/10 = 18/20
you multiply the 9 by the 20 and you get 180,
you multiply the 18 by the 10 and you also get 180. Pretty simple right?
Let's take a look at another one.
x/9 = 10/5
You multiply the x by the 5 and you get 5x,
you multiply the 9 by the 10 you get 90.
Now our equation is 5x = 90.
Now you divide the 5x by 5 and the 90 by 5.
x turns out to be 18.