The third quartile of this data set is B. 24.
Answer:

Step-by-step explanation:
see the attached figure with letters to better understand the problem
we know that
In the right triangle ABD

In the right triangle BCD

remember that
m< BAD=m<CBD
therefore

equate the equations



If you mean proof that the opposite sides of a parallelogram are equal, you draw a diagonal then prove that the 2 triangles formed are congruent.