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. For a given input n, find the sum of multiples of 3 or five between 1 and n. Test cases 10 => 23 100 => 2318 1000 => 233168 This challenge was first a...

Concentration (or Memory or Match) is a game where players pick pairs of cards and try to find matches. The rules are as follows: Two of each number from 1 to N are added to the deck. The deck is shuffled and placed face-down (hidden). The player selects a card. The card is revealed. The pla...

The sum of the squares of the first ten natural numbers is, 1² + 2² + ... + 10² = 385 The square of the sum of the first ten natural numbers is, (1 + 2 + ... + 10)2 = 55² = 3025 Hence the difference between the sum of the squares of the first ten natural numbers and the square of the sum is ...

I think the Collatz Conjecture is already well-known. But what if we invert the rules? Start with an integer n > 1. Repeat the following steps: If n is even, multiply it by 3 and add 1. If n is odd, subtract 1 and divide it by 2. Stop when it reaches 0 Print the iterated numbers. Test case...