ID Description / Title Solved By 1 Multiples of 3 and 5 2 Even Fibonacci numbers 3 Largest prime factor 4 Largest palindrome product 5 Smallest multiple 6 Sum square difference 7 10001st prime 8 Largest product in a series 9 Special Pythagorean triplet 10 Summation of primes 11 Largest product in a grid 12 Highly divisible triangular number 13 Large sum 14 Longest Collatz sequence 15 Lattice paths 16 Power digit sum 17 Number letter counts 18 Maximum path sum I 19 Counting Sundays 20 Factorial digit sum 21 Amicable numbers 22 Names scores 23 Non-abundant sums 24 Lexicographic permutations 25 1000-digit Fibonacci number 26 Reciprocal cycles 27 Quadratic primes 28 Number spiral diagonals 29 Distinct powers 30 Digit fifth powers 31 Coin sums 32 Pandigital products 33 Digit cancelling fractions 34 Digit factorials 35 Circular primes 36 Double-base palindromes 37 Truncatable primes 38 Pandigital multiples 39 Integer right triangles 40 Champernowne's constant 41 Pandigital prime 42 Coded triangle numbers 43 Sub-string divisibility 44 Pentagon numbers 45 Triangular, pentagonal, and hexagonal 46 Goldbach's other conjecture 47 Distinct primes factors 48 Self powers 49 Prime permutations 50 Consecutive prime sum