When we look at 16/18, we need to think of a number that goes evenly into both 16 and 18. Let's do 2.
16/2 = 8
18/2= 9
Your new fraction is 8/9
Can we reduce that anymore? There isn't one number, besides one, that will go into both 8 and 9 evenly, so therefore, we are done. 8/9 is your answer.