(-15又2/3)-[(-13又2/3)+(-31又2/15)+(+14)]
=(-15又2/3)-(-13又2/3)-(-31又2/15)-14
=(-15又2/3)+(13又2/3)+(31又2/15)-14
=-2+(31又2/15)-14
=31又2/15-16
=15又2/15