The interior diagonal is the hypotenuse of the triangle in
which one side is the edge of the cube
and the other side is the diagonal on a face of that cube.
The cube edge is 15 units and the length of diagonal of a face is 15*Ö(2).
That means the interior diagonal of the cube is Ö(15)2
+ (15*Ö(2))2)=Ö(675)=15Ö(3)=25.98