ID	Title	Solved by	Difficulty
1	Multiples of 3 or 5	968630	5%
2	Even Fibonacci numbers	772730	5%
3	Largest prime factor	556069	5%
6	Sum square difference	498564	5%
5	Smallest multiple	495549	5%
4	Largest palindrome product	492178	5%
7	10001st prime	426858	5%
9	Special Pythagorean triplet	362853	5%
8	Largest product in a series	357805	5%
10	Summation of primes	332577	5%
11	Largest product in a grid	238908	5%
16	Power digit sum	233303	5%
14	Longest Collatz sequence	231075	5%
13	Large sum	230801	5%
12	Highly divisible triangular number	225493	5%
20	Factorial digit sum	202010	5%
15	Lattice paths	190748	5%
25	1000-digit Fibonacci number	158637	5%
17	Number letter counts	154837	5%
21	Amicable numbers	149538	5%
18	Maximum path sum I	148231	5%
19	Counting Sundays	137720	5%
22	Names scores	137531	5%
24	Lexicographic permutations	117285	5%
48	Self powers	114337	5%
30	Digit fifth powers	112482	5%
28	Number spiral diagonals	110459	5%
29	Distinct powers	107885	5%
23	Non-abundant sums	106742	5%
67	Maximum path sum II	97707	5%
34	Digit factorials	96375	5%
36	Double-base palindromes	90805	5%
27	Quadratic primes	89543	5%
31	Coin sums	87177	5%
35	Circular primes	86642	5%
26	Reciprocal cycles	86089	5%
40	Champernowne's constant	81935	5%
42	Coded triangle numbers	76092	5%
37	Truncatable primes	75315	5%
39	Integer right triangles	74942	5%
33	Digit cancelling fractions	73468	5%
32	Pandigital products	73072	5%
45	Triangular, pentagonal, and hexagonal	72550	5%
41	Pandigital prime	69753	5%
52	Permuted multiples	66623	5%
38	Pandigital multiples	64591	5%
50	Consecutive prime sum	63927	5%
46	Goldbach's other conjecture	63139	5%
43	Sub-string divisibility	61345	5%
53	Combinatoric selections	60220	5%
44	Pentagon numbers	60009	5%
56	Powerful digit sum	59427	5%
47	Distinct primes factors	59419	5%
49	Prime permutations	59316	5%
55	Lychrel numbers	55012	5%
63	Powerful digit counts	44173	5%
97	Large non-Mersenne prime	43921	5%
57	Square root convergents	42641	5%
92	Square digit chains	42351	5%
59	XOR decryption	42195	5%
79	Passcode derivation	41738	5%
58	Spiral primes	41192	5%
206	Concealed Square	24662	5%
686	Powers of Two	2962	5%
700	Eulercoin	2662	5%
719	Number Splitting	2566	5%
684	Inverse Digit Sum	2320	5%
751	Concatenation Coincidence	1550	5%
54	Poker hands	37000	10%
69	Totient maximum	35633	10%
81	Path sum: two ways	34967	10%
99	Largest exponential	31147	10%
71	Ordered fractions	29951	10%
76	Counting summations	29152	10%
357	Prime generating integers	8119	10%
387	Harshad Numbers	4680	10%
650	Divisors of Binomial Product	1535	10%
757	Stealthy Numbers	832	10%
745	Sum of Squares II	825	10%
51	Prime digit replacements	34812	15%
62	Cubic permutations	32241	15%
65	Convergents of e	30570	15%
74	Digit factorial chains	27340	15%
73	Counting fractions in a range	25566	15%
85	Counting rectangles	25180	15%
112	Bouncy numbers	24989	15%
102	Triangle containment	22343	15%
345	Matrix Sum	5657	15%
347	Largest integer divisible by two primes	4550	15%
346	Strong Repunits	4308	15%
381	(prime-k) factorial	4196	15%
500	Problem 500!!!	3943	15%
549	Divisibility of factorials	2655	15%
622	Riffle Shuffles	1661	15%
694	Cube-full Divisors	856	15%
749	Near Power Sums	541	15%
60	Prime pair sets	27411	20%
61	Cyclical figurate numbers	26153	20%
72	Counting fractions	22856	20%
70	Totient permutation	22788	20%
64	Odd period square roots	22683	20%
89	Roman numerals	21569	20%
82	Path sum: three ways	21526	20%
87	Prime power triples	21310	20%
80	Square root digital expansion	20249	20%
145	How many reversible numbers are there below one-billion?	16827	20%
429	Sum of squares of unitary divisors	2495	20%
407	Idempotents	2411	20%
601	Divisibility streaks	1966	20%
516	5-smooth totients	1535	20%
618	Numbers with a given prime factor sum	985	20%
668	Square root smooth Numbers	961	20%
609	π sequences	892	20%
659	Largest prime	847	20%
704	Factors of Two in Binomial Coefficients	787	20%
688	Piles of Plates	672	20%
754	Product of Gauss Factorials	520	20%
68	Magic 5-gon ring	21255	25%
66	Diophantine equation	20151	25%
77	Prime summations	19577	25%
83	Path sum: four ways	18509	25%
75	Singular integer right triangles	18399	25%
96	Su Doku	17496	25%
104	Pandigital Fibonacci ends	16429	25%
120	Square remainders	13966	25%
124	Ordered radicals	13870	25%
125	Palindromic sums	13679	25%
179	Consecutive positive divisors	11289	25%
187	Semiprimes	11223	25%
203	Squarefree Binomial Coefficients	9172	25%
348	Sum of a square and a cube	3012	25%
401	Sum of squares of divisors	2524	25%
512	Sums of totients of powers	1460	25%
425	Prime connection	1431	25%
479	Roots on the Rise	1263	25%
531	Chinese leftovers	1097	25%
571	Super Pandigital Numbers	1067	25%
675	2^{\omega(n)}	674	25%
752	Powers of 1+\sqrt 7	510	25%
78	Coin partitions	17255	30%
100	Arranged probability	16576	30%
95	Amicable chains	14576	30%
108	Diophantine reciprocals I	12802	30%
119	Digit power sum	12530	30%
123	Prime square remainders	11656	30%
173	Using up to one million tiles how many different "hollow" square laminae can be formed?	9119	30%
204	Generalised Hamming Numbers	7327	30%
293	Pseudo-Fortunate Numbers	2952	30%
451	Modular inverses	1394	30%
485	Maximum number of divisors	1127	30%
443	GCD sequence	1106	30%
435	Polynomials of Fibonacci numbers	1091	30%
581	47-smooth triangular numbers	819	30%
784	Reciprocal Pairs	284	30%
94	Almost equilateral triangles	12588	35%
93	Arithmetic expressions	12028	35%
98	Anagramic squares	11603	35%
243	Resilience	9549	35%
191	Prize Strings	7083	35%
188	The hyperexponentiation of a number	6391	35%
303	Multiples with small digits	3603	35%
277	A Modified Collatz sequence	3217	35%
349	Langton's ant	1900	35%
288	An enormous factorial	1633	35%
343	Fractional Sequences	1391	35%
463	A weird recurrence relation	1062	35%
417	Reciprocal cycles II	838	35%
565	Divisibility of sum of divisors	590	35%
625	Gcd sum	564	35%
621	Expressing an integer as the sum of triangular numbers	530	35%
545	Faulhaber's Formulas	510	35%
110	Diophantine reciprocals II	8167	40%
131	Prime cube partnership	7410	40%
174	Counting the number of "hollow" square laminae that can form one, two, three, ... distinct arrangements	5852	40%
231	The prime factorisation of binomial coefficients	5436	40%
214	Totient Chains	5194	40%
235	An Arithmetic Geometric sequence	4986	40%
207	Integer partition equations	4707	40%
304	Primonacci	2142	40%
365	A huge binomial coefficient	1229	40%
487	Sums of power sums	627	40%
634	Numbers of the form a^2b^3	573	40%
748	Upside down Diophantine equation	268	40%
111	Primes with runs	7303	45%
134	Prime pair connection	7048	45%
118	Pandigital prime sets	7018	45%
135	Same differences	6415	45%
132	Large repunit factors	6363	45%
129	Repunit divisibility	6319	45%
130	Composites with prime repunit property	5938	45%
138	Special isosceles triangles	5847	45%
142	Perfect Square Collection	5831	45%
197	Investigating the behaviour of a recursively defined sequence	4888	45%
183	Maximum product of parts	4661	45%
225	Tribonacci non-divisors	4443	45%
216	Investigating the primality of numbers of the form 2n2-1	4058	45%
291	Panaitopol Primes	1420	45%
642	Sum of largest prime factors	335	45%
127	abc-hits	6265	50%
136	Singleton difference	5687	50%
133	Repunit nonfactors	5543	50%
137	Fibonacci golden nuggets	5539	50%
146	Investigating a Prime Pattern	5059	50%
169	Exploring the number of different ways a number can be expressed as a sum of powers of 2	5045	50%
211	Divisor Square Sum	4160	50%
320	Factorials divisible by a huge integer	849	50%
521	Smallest prime factor	715	50%
708	Twos are all you need	282	50%
140	Modified Fibonacci golden nuggets	4343	55%
193	Squarefree Numbers	3313	55%
678	Fermat-like Equations	221	55%
160	Factorial trailing digits	3486	60%
182	RSA encryption	2589	60%
271	Modular Cubes, part 1	2346	60%
196	Prime triplets	2555	65%
221	Alexandrian Integers	2052	65%
273	Sum of Squares	1360	70%
248	Numbers for which Euler’s totient function equals 13!	1283	70%
379	Least common multiple count	505	70%
192	Best Approximations	1583	75%
795	Alternating GCD Sum	195	-