60. As shown in the figure: In the tetrahedron $P-ABC$, $\angle APB=35^{\circ}, \angle BPC=25^{\circ}, \angle CPA=30^{\circ}$, points $M$ and $N$ are on edge $PB$, and $PN=9, PM=12$. One end of a thin string is fixed at point $M$, and then it is tightly wound around the side of the tetrahedron, exactly reaching point $N$. The length of this thin string is
Reference answer: 15