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

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