Question

T(8,15) is the midpoint of CD. The coordinates of D are (8,20). What are the coordinates of C?

Answer 1

The coordinates of C must be (8,10).

If you plug in the values given in the midpoint formula
$8= \frac{8+(8)}{2}$
 and $15= \frac{20+(10)}{2}$

it matches perfectly with the provided midpoint

Answer 2

The mid point is 5 away from d so it should be five away from c. 8,5