check the picture below.
keeping in mind that the diagonals in a rhombus always meet at right-angles, thus all angles at point O are all right-angles.
now, notice that the AC angle bisector, cuts the angle at A in two equal halves, and if you look at the triangle BOC, there is a 15.5° angle, and a right-angle, and well, you already know what the last one is.
