now since you know the definitions, i don't suppose any of this will come as a surprise to you
so lets do this.
lets make up some data, for simplicity, we will take 5 values. i will write our values in ascending order, and an unknown x
13, 13, 15, 15, x
now we want:
now solve for x
can you see why i choose the data like that? do you see why it works?