Question 990594
The diagonals are perpendicular to each other and bisect each other.
Therefore, the two legs of the right triangle are the sides of each diagonal and the side CD is the hypotenuse.
The angle is the adjacent side (10, half of the diagonal) divided by 15, the hypotenuse.  That is the cosine, so cos of x=10/15
The arc cos of (10/15) is 48.2 degrees.