Sum square difference
The sum of the squares of the first ten natural numbers is,
1^2 + 2^2 + … + 10^2 = 385
The square of the sum of the first ten natural numbers is,
(1 + 2 + … + 10)^2 = 55^2 = 3025
Hence the difference between the sum of the squares...
Multiples of 3 and 5
If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
Find the sum of all the multiples of 3 or 5 below 1000.
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Digit factorials.
145 is a curious number, as 1! + 4! + 5! = 1 + 24 + 120 = 145.
Find the sum of all numbers which are equal to the sum of the factorial of their digits.
Note: as 1! = 1 and 2! = 2 are not sums they are not included.
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