Hi Jaden,
I'm going to use decimal notation to help minimize any confusion here.
hence, 16 3/4 = 16.75 and 18 1/2 = 18.5
The difference in the first two numbers is : 16.75 - 15 = 1.75
The difference in the 3rd and the 2nd numbers : 18.5 - 16.75 = 1.75
So it appears that we are adding 1.75 to each number to get the next value.
So the next 2 numbers in this sequence would be
18.5 + 1.75 = 20.25
20.25 + 1.75 = 22.0
Glenn