ID	Title	Solved by	Difficulty
1	Multiples of 3 or 5	969886	5%
2	Even Fibonacci numbers	774168	5%
3	Largest prime factor	557702	5%
6	Sum square difference	500165	5%
5	Smallest multiple	497139	5%
4	Largest palindrome product	493800	5%
7	10001st prime	428427	5%
9	Special Pythagorean triplet	364422	5%
8	Largest product in a series	359417	5%
10	Summation of primes	334172	5%
11	Largest product in a grid	240398	5%
16	Power digit sum	234844	5%
14	Longest Collatz sequence	232624	5%
13	Large sum	232366	5%
12	Highly divisible triangular number	227038	5%
20	Factorial digit sum	203435	5%
15	Lattice paths	192207	5%
25	1000-digit Fibonacci number	160004	5%
17	Number letter counts	156221	5%
21	Amicable numbers	150850	5%
18	Maximum path sum I	149569	5%
19	Counting Sundays	138967	5%
22	Names scores	138836	5%
24	Lexicographic permutations	118527	5%
48	Self powers	115422	5%
30	Digit fifth powers	113627	5%
28	Number spiral diagonals	111564	5%
29	Distinct powers	109051	5%
23	Non-abundant sums	107962	5%
67	Maximum path sum II	98719	5%
34	Digit factorials	97443	5%
36	Double-base palindromes	91819	5%
27	Quadratic primes	90629	5%
31	Coin sums	88278	5%
35	Circular primes	87709	5%
26	Reciprocal cycles	87206	5%
40	Champernowne's constant	82906	5%
42	Coded triangle numbers	77009	5%
37	Truncatable primes	76286	5%
39	Integer right triangles	75880	5%
33	Digit cancelling fractions	74416	5%
32	Pandigital products	74053	5%
45	Triangular, pentagonal, and hexagonal	73467	5%
41	Pandigital prime	70694	5%
52	Permuted multiples	67521	5%
38	Pandigital multiples	65477	5%
50	Consecutive prime sum	64857	5%
46	Goldbach's other conjecture	64024	5%
43	Sub-string divisibility	62221	5%
53	Combinatoric selections	61054	5%
44	Pentagon numbers	60888	5%
47	Distinct primes factors	60303	5%
56	Powerful digit sum	60248	5%
49	Prime permutations	60191	5%
55	Lychrel numbers	55797	5%
63	Powerful digit counts	44851	5%
97	Large non-Mersenne prime	44577	5%
57	Square root convergents	43354	5%
92	Square digit chains	43018	5%
59	XOR decryption	42866	5%
79	Passcode derivation	42338	5%
58	Spiral primes	41852	5%
206	Concealed Square	25035	5%
686	Powers of Two	3248	5%
719	Number Splitting	3205	5%
700	Eulercoin	3026	5%
684	Inverse Digit Sum	2511	5%
751	Concatenation Coincidence	1910	5%
808	Reversible prime squares	1477	5%
800	Hybrid Integers	1262	5%
836	A Bold Proposition	1035	5%
54	Poker hands	37674	10%
69	Totient maximum	36240	10%
81	Path sum: two ways	35565	10%
99	Largest exponential	31691	10%
71	Ordered fractions	30502	10%
76	Counting summations	29674	10%
357	Prime generating integers	8321	10%
387	Harshad Numbers	4781	10%
650	Divisors of Binomial Product	1615	10%
757	Stealthy Numbers	1020	10%
745	Sum of Squares II	978	10%
51	Prime digit replacements	35517	15%
62	Cubic permutations	32886	15%
65	Convergents of e	31123	15%
74	Digit factorial chains	27841	15%
73	Counting fractions in a range	26032	15%
85	Counting rectangles	25692	15%
112	Bouncy numbers	25428	15%
102	Triangle containment	22750	15%
345	Matrix Sum	5799	15%
347	Largest integer divisible by two primes	4671	15%
346	Strong Repunits	4415	15%
381	(prime-k) factorial	4313	15%
500	Problem 500!!!	4072	15%
549	Divisibility of factorials	2749	15%
622	Riffle Shuffles	1735	15%
694	Cube-full Divisors	922	15%
749	Near Power Sums	608	15%
60	Prime pair sets	28027	20%
61	Cyclical figurate numbers	26686	20%
72	Counting fractions	23338	20%
70	Totient permutation	23273	20%
64	Odd period square roots	23169	20%
89	Roman numerals	21994	20%
82	Path sum: three ways	21956	20%
87	Prime power triples	21722	20%
80	Square root digital expansion	20680	20%
145	How many reversible numbers are there below one-billion?	17162	20%
429	Sum of squares of unitary divisors	2570	20%
407	Idempotents	2492	20%
601	Divisibility streaks	2048	20%
518	Prime triples and geometric sequences	1620	20%
516	5-smooth totients	1583	20%
618	Numbers with a given prime factor sum	1033	20%
668	Square root smooth Numbers	1025	20%
609	π sequences	929	20%
659	Largest prime	907	20%
704	Factors of Two in Binomial Coefficients	856	20%
688	Piles of Plates	728	20%
754	Product of Gauss Factorials	603	20%
68	Magic 5-gon ring	21726	25%
66	Diophantine equation	20601	25%
77	Prime summations	20025	25%
83	Path sum: four ways	18923	25%
75	Singular integer right triangles	18808	25%
96	Su Doku	17897	25%
104	Pandigital Fibonacci ends	16771	25%
120	Square remainders	14256	25%
124	Ordered radicals	14165	25%
125	Palindromic sums	13973	25%
179	Consecutive positive divisors	11513	25%
187	Semiprimes	11428	25%
203	Squarefree Binomial Coefficients	9328	25%
348	Sum of a square and a cube	3101	25%
401	Sum of squares of divisors	2599	25%
512	Sums of totients of powers	1517	25%
425	Prime connection	1491	25%
479	Roots on the Rise	1305	25%
531	Chinese leftovers	1149	25%
571	Super Pandigital Numbers	1110	25%
675	2^{\omega(n)}	712	25%
752	Powers of 1+\sqrt 7	573	25%
78	Coin partitions	17679	30%
100	Arranged probability	16995	30%
95	Amicable chains	14948	30%
108	Diophantine reciprocals I	13077	30%
119	Digit power sum	12784	30%
123	Prime square remainders	11916	30%
173	Using up to one million tiles how many different "hollow" square laminae can be formed?	9299	30%
204	Generalised Hamming Numbers	7493	30%
293	Pseudo-Fortunate Numbers	3038	30%
451	Modular inverses	1459	30%
485	Maximum number of divisors	1170	30%
443	GCD sequence	1138	30%
435	Polynomials of Fibonacci numbers	1133	30%
581	47-smooth triangular numbers	855	30%
784	Reciprocal Pairs	344	30%
795	Alternating GCD Sum	280	30%
94	Almost equilateral triangles	12920	35%
93	Arithmetic expressions	12331	35%
98	Anagramic squares	11923	35%
243	Resilience	9752	35%
191	Prize Strings	7235	35%
188	The hyperexponentiation of a number	6507	35%
303	Multiples with small digits	3690	35%
277	A Modified Collatz sequence	3297	35%
349	Langton's ant	1956	35%
288	An enormous factorial	1681	35%
343	Fractional Sequences	1429	35%
463	A weird recurrence relation	1101	35%
417	Reciprocal cycles II	879	35%
565	Divisibility of sum of divisors	618	35%
625	Gcd sum	603	35%
621	Expressing an integer as the sum of triangular numbers	557	35%
545	Faulhaber's Formulas	528	35%
110	Diophantine reciprocals II	8391	40%
131	Prime cube partnership	7586	40%
174	Counting the number of "hollow" square laminae that can form one, two, three, ... distinct arrangements	5990	40%
231	The prime factorisation of binomial coefficients	5547	40%
214	Totient Chains	5316	40%
235	An Arithmetic Geometric sequence	5082	40%
207	Integer partition equations	4798	40%
304	Primonacci	2219	40%
365	A huge binomial coefficient	1280	40%
487	Sums of power sums	654	40%
634	Numbers of the form a^2b^3	603	40%
748	Upside down Diophantine equation	282	40%
111	Primes with runs	7523	45%
118	Pandigital prime sets	7243	45%
134	Prime pair connection	7227	45%
135	Same differences	6586	45%
132	Large repunit factors	6547	45%
129	Repunit divisibility	6499	45%
130	Composites with prime repunit property	6117	45%
138	Special isosceles triangles	6006	45%
142	Perfect Square Collection	5992	45%
197	Investigating the behaviour of a recursively defined sequence	5004	45%
183	Maximum product of parts	4778	45%
225	Tribonacci non-divisors	4548	45%
216	Investigating the primality of numbers of the form 2n2-1	4158	45%
291	Panaitopol Primes	1473	45%
642	Sum of largest prime factors	350	45%
127	abc-hits	6451	50%
136	Singleton difference	5856	50%
133	Repunit nonfactors	5711	50%
137	Fibonacci golden nuggets	5691	50%
146	Investigating a Prime Pattern	5217	50%
169	Exploring the number of different ways a number can be expressed as a sum of powers of 2	5178	50%
211	Divisor Square Sum	4266	50%
320	Factorials divisible by a huge integer	883	50%
521	Smallest prime factor	741	50%
708	Twos are all you need	295	50%
140	Modified Fibonacci golden nuggets	4473	55%
193	Squarefree Numbers	3435	55%
678	Fermat-like Equations	229	55%
160	Factorial trailing digits	3598	60%
182	RSA encryption	2658	60%
271	Modular Cubes, part 1	2422	60%
196	Prime triplets	2642	65%
221	Alexandrian Integers	2121	65%
273	Sum of Squares	1407	70%
248	Numbers for which Euler’s totient function equals 13!	1320	70%
379	Least common multiple count	524	70%
192	Best Approximations	1659	75%