If a card is drawn from a well shuffled pack of cards, the probability of drawing a spade or a king is -

$$P\left( {S \cup K} \right) = $$   $$P\left( S \right) + $$  $$P\left( K \right) - $$  $$P\left( {S \cap K} \right),$$   (where S denotes spade and K denotes king.)
$$\eqalign{ & P\left( {S \cup K} \right) = \frac{{13}}{{52}} + \frac{4}{{52}} - \frac{1}{{52}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{4}{{13}} \cr} $$