The only way in which an ace and a spade can both be drawn is to draw the ace of spades. There is only one ace of spades, so:

p(ace and spade) = 1/52 .

The probability of an ace or a spade can be computed as:

p(ace)+p(spade)-p(ace and spade) = 4/52 + 13/52 - 1/52 = 16/52 = 4/13.

Consider the probability of rolling a die twice and getting a 6 on at least one of the rolls. The events are defined in the following way:

Event A: 6 on the first roll: p(A) = 1/6

Event B: 6 on the second roll: p(B) = 1/6

p(A and B) = 1/6 x 1/6

p(A or B) = 1/6 + 1/6 - 1/6 x 1/6 = 11/36

The same answer can be computed using the following admittedly convoluted approach: Getting a 6 on either roll is the same thing as not getting a number from 1 to 5 on both rolls. This is equal to: 1 - p(1 to 5 on both rolls).