When a single card is drawn from an ordinary 52 card deck, find the probability it will be the following; A red Card: There are 26 red cards out of 52 cards, so the probability is P(R) = 26/52 = 1/2 An Ace: There are 4 aces out of 52 cards, so the probability is P(A) = 4/52 = 1/13 A heart or a king: P(H or K) = P(H)+P(K)-P(H and K) There are 13 hearts out of 52 cards, so P(H) = 13/52 = 1/4 There are 4 kings out of 52 cards, so P(K) = 4/52 = 1/13 There is 1 card which is a heart and a king (the king of hearts), so P(H and K) = 1/52, so P(H or K) = P(H)+P(K)-P(H and K) = 1/4+1/13-1/52 = 13/52+4/52-1/52 = 16/52 = 4/13 A club, given it is black: P(C|B) = P(C and B)/P(B) There are 13 cards out of 52 which Clubs and Blacks so P(C and B) = 13/52 = 1/4 There are 26 cards out of 52 which are black, so P(B) = 26/52 = 1/2 P(C|B) = P(C and B)/P(B) = (1/4)/(1/2) = (1/4)(2/1) = 1/2 An ace or a card above a jack Hmmm! This will depend on whether and ace is considered higher or lower than a jack. It is usually higher, but sometimes it's lower. I'll do both cases: Case 1. An ace is considered higher than a jack: P(A or Above Jack) = P(A) + P(Above Jack) - P(A and Above Jack) There are 4 aces, so P(A) = 4/52 = 1/13 There are 4 queens, 4 kings and 4 aces which are above jack, so P(Above jack) = 12/52 = 3/13 All 4 aces are above Jack so P(A and Above Jack) = 4/52 = 1/13, so P(A or Above Jack) = P(A) + P(Above Jack) - P(A and Above Jack) = 1/13 + 3/13 - 1/13 = 3/13 Case 2. An ace is considered lower than a jack: P(A or Above Jack) = P(A) + P(Above Jack) - P(A and Above Jack) There are 4 aces, so P(A) = 4/52 = 1/13 There are 4 queens and 4 kings which are above jack, so P(Above jack) = 8/52 = 2/13 None of the 4 aces are above Jack so P(A and Above Jack) = 0/52 = 0, so P(A or Above Jack) = P(A) + P(Above Jack) - P(A and Above Jack) = 1/13 + 3/13 - 0 = 4/13 A queen, given it is a spade: P(Q|S) = P(Q and S)/P(S) There is only one card that is a queen and a spade, namely the queen of spades, so P(Q and S) = 1/52. There are 13 spades, so P(S) = 13/52 = 1/4, so P(Q|S) = P(Q and S)/P(S) = (1/52)/(1/4) = (1/52)(4/1) = 1/13 A 5, given it is a face card P(5|F) = P(5 and F)/P(F) Since a 5 is not a face card, P(5 and F) = 0 Since there are 4 Jacks, 4 queens, and 4 kings which are face cards, P(F) = 12/52 = 3/13, so P(5|F) = P(5 and F)/P(F) = 0/(3/13) = 0(13/3) = 0 --------------- Now assume that the two cards are drawn without replacement. Find the probability of each of the following events. Both are aces: P(A 1st and A 2nd) = P(A 1st)·P(A 2nd) = (4/52)(3/51) = 1/221 One is a king and one is an ace: P[(A 1st and K 2nd) or (K 1st and A 2nd)] = P(A 1st and K 2nd) + P(K 1st and A 2nd) = P(A 1st)·P(K 2nd) + P(K 1st)·P(A 2nd) = (4/52)(4/51) + (4/52)(4/51) = 8/663 Neither one is an ace: There are 48 non-aces, so P(non-A 1st and non-A 2nd) = P(non-A 1st)·P(non-A 2nd) = (48/52)(47/51) = 188/221 Both are red: P(R 1st and R 2nd) = P(R 1st)·P(R 2nd) = (26/52)(25/51) = 25/102 Both are face cards: P(F 1st and F 2nd) = P(F 1st)·P(F 2nd) = (12/52)(11/51) = 11/221 A face card Here it is easier to use the complement event, P(one is a face card) = 1 - P(both are non-face card). (There are 12 face cards and 40 non-face cards) 1 - P(non-F 1st and non-F 2nd) = 1 - P(non-F 1st)·P(non-F 2nd) = 1 - (40/52)(39/51) = 7/17 This could have also been worked a longer way without using the complement event: P[(F 1st and F 2nd) or (F 1st and non-F 2nd) or (non-F 1st and F 2nd)] = P(F 1st and F 2nd) + P(F 1st and non-F 2nd) + P(non-F 1st and F 2nd) = P(F 1st)·P(F 2nd) + P(F 1st)·P(non-F 2nd) + P(non-F 1st)·P(F 2nd) = (12/52)(11/51) + (12/52)(40/51) + (40/52)(12/52) = 11/221 + 40/221 + 40/221 = 7/17 A black 2 or a face card This one is also much easier to work using the complement event: There are 2 black 2's and 12 face cards or 14 which are one or the other. That leaves 38 cards which are neither. P(B2 or F) = 1 - P(neither 1st and neither 2nd) = P(neither 1st)·P(neither 2nd) = (38/52)·(37/51) = 703/1326 This could have also been done a much longer way without using the complement event, but I will not bother with that. Edwin