The answer is 20. You have to use the Pythagorean theorem (A^2+B^2=C^2) to find the length of the hypotenuse.
32^2+24^2=C^2
1024+576=C^2
1600=C^2
Take the square root of both sides
40=C
Since the midpoint is the middle, you divide the length of the hypotenuse by 2 and get the answer (20)