You can put this solution on YOUR website! There's a whole big complicated proportion for this, or I could just tell you that since 9 and 18 are both on the bottom, since 9 is half of 18, whatever is over 18 will be in half over 9.
so if 10 is over 18 then 5 should be over 9.
But if you wanted to do proportions, you would cross multiply, end up with 18k=90, divide by 18 on both sides and still get 5.
Hope I helped! :)
-Rosey