Rank	Difficulty	Solved	Question	Date
1 1	1	1	Bowling Ball Bowling Ball	2016-01-30 2016-01-30
2 2	1	1	You vs. Train You vs. Train	2016-01-30 2016-01-30
3 3	1	1	How many ways How many ways	2016-08-08 2016-08-08
4 4	2	1	Excited Dog Excited Dog	2016-01-30 2016-01-30
5 5	2	1	Cursed Temple Cursed Temple	2016-02-28 2016-02-28
6 6	2	1	Red and Blue Marbles Red and Blue Marbles	2016-07-15 2016-07-15
7 7	2	1	How Many Sons How Many Sons	2018-06-17 2018-06-17
8 8	2	1	Crossing ladders Crossing ladders	2016-01-17 2016-01-17
9 9	3	1	John and Julia John and Julia	2016-01-30 2016-01-30
10 10	3	1	How many ways level 2 How many ways level 2	2016-08-08 2016-08-08
11 11	3	1	Red Black Cards Red Black Cards	2016-01-30 2016-01-30
12 12	3	1	Guess the Password Guess the Password	2016-07-15 2016-07-15
13 13	3	1	Birthday Problem Birthday Problem	2016-05-26 2016-05-26
14 14	3	1	NBA Finals NBA Finals	2016-08-08 2016-08-08
15 15	3	1	Two Egg Drop Two Egg Drop	2016-08-08 2016-08-08
16 16	3	1	Occupied! Occupied!	2017-10-24 2017-10-24
17 17	4	1	Coin flipping Coin flipping	2016-01-30 2016-01-30
18 18	4	1	Survival of the species Survival of the species	2017-02-12 2017-02-12
19 19	4	1	Hole in the Wall Hole in the Wall	2016-01-31 2016-01-31
20 20	4	1	Lockers Lockers	2016-01-30 2016-01-30
21 21	4	1	3 Heads and 5 Hats 3 Heads and 5 Hats	2016-01-30 2016-01-30
22 22	4	1	A Deadly Cure A Deadly Cure	2016-01-30 2016-01-30
23 23	4	1	Bloodthirsty Pirates Bloodthirsty Pirates	2016-01-31 2016-01-31
24 24	4	1	Traffic Jams Traffic Jams	2016-02-06 2016-02-06
25 25	4	1	Minimum number of weights Minimum number of weights	2016-01-30 2016-01-30
26 26	4	0	Hot Potato Hot Potato	2017-10-24 2017-10-24
27 27	4	1	Linear Prisoners Linear Prisoners	2016-04-30 2016-04-30
28 28	4	0	Prison with Levers Prison with Levers	2017-10-26 2017-10-26
29 29	5	1	12 balls 12 balls	2016-07-15 2016-07-15
30 30	5	1	All wrong All wrong	2016-02-06 2016-02-06
31 31	5	1	The Cliff The Cliff	2016-01-30 2016-01-30
32 32	5	1	Elevator Buttons Elevator Buttons	2016-05-26 2016-05-26
33 33	5	1	Linear prisoners... with more hats Linear prisoners... with more hats	2018-07-05 2018-07-05
34 34	5	1	All Mixed Up All Mixed Up	2017-01-21 2017-01-21
35 35	5	0	Rock, Paper, Scissors, Double Scissors Rock, Paper, Scissors, Double Scissors	2017-10-15 2017-10-15
36 36	5	1	Presidential Streaks Presidential Streaks	2017-01-14 2017-01-14
37 37	5	0	Cube Creature Cube Creature	2016-01-31 2016-01-31
38 38	5	1	Two Mathematicians an a Coin Two Mathematicians an a Coin	2018-05-24 2018-05-24
39 39	6	1	Truel Truel	2016-01-31 2016-01-31
40 40	6	0	Highest Card Highest Card	2017-06-13 2017-06-13
41 41	6	0	Drunken Square Drunken Square	2016-01-31 2016-01-31
42 42	6	1	How many teams How many teams	2016-05-03 2016-05-03
43 43	6	1	Reverse Birthday Problem Reverse Birthday Problem	2016-05-26 2016-05-26
44 44	6	0	Colorful Dwarves Colorful Dwarves	2016-01-31 2016-01-31
45 45	6	1	Five Card Magic Trick Five Card Magic Trick	2017-01-08 2017-01-08
46 46	6	1	Neurotic Basketball Player Neurotic Basketball Player	2016-01-17 2016-01-17
47 47	7	1	abracadabra abracadabra	2016-01-30 2016-01-30
48 48	7	0	Guess the Answers Guess the Answers	2016-05-03 2016-05-03
49 49	8	1	Higher or Lower Higher or Lower	2016-02-06 2016-02-06
50 50	8	1	Moser's circle problem Moser's circle problem	2018-05-24 2018-05-24
51 51	8	1	64 quarters 64 quarters	2016-01-30 2016-01-30
52 52	8	1	Tic Tac Toe Tic Tac Toe	2016-01-30 2016-01-30
53 53	8	1	Circular Prisoners Circular Prisoners	2016-07-18 2016-07-18
54 54	9	1	Two envelopes problem revisited Two envelopes problem revisited	2016-01-30 2016-01-30
55 55	9	1	100 prisoners 100 prisoners	2016-01-30 2016-01-30

Other recommended sites:

• 538's The Riddler
• wu : riddles
• Ultimate Puzzle Size
• Mosteller Fifty Challenging Problems in Probability
• Sam Steingold's list of puzzles