Translate the following inference to logic notation: For all x, x_7 will write a letter if and only if x_7 will go running. For all x, x_7 will go running if and only if x_7 will take a plane. For all x, x_7 will not write a letter. Therefore for all x, x_7 will not take a plane.
forall x: P(x) <-> Q(x). forall x: Q(x) <-> R(x). forall x: ~P(x). Therefore forall x: ~R(x).