Translate the following inference to logic notation: For all x, if x_13 writes a letter, then x_13 is playing squash. For all x, x_13 will write a letter or x_13 is not a cashier. For all x, if x_13 plays squash, then x_13 is writing a letter. There is at least one x for which x_13 will not play squash. Therefore there is at least one x for which x_13 is not a cashier.
forall x: P(x) -> Q(x). forall x: P(x) or ~R(x). forall x: Q(x) -> P(x). exists x: ~Q(x). Therefore exists x: ~R(x).