What is Project Euler

Project Euler is a website with a collection of programming and mathematical problems. Each challenge varies in difficulty and there are over 600 challenges. I highly recommend anyone interested in puzzles and programming to create an account and start solving. About Project Euler

** Stop here if you have not yet tried and solved Problem 1. **

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Problem 1

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.

class Problem1
  def question
    "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."
  end

  def sum_multiples_3_and_5 n
    return 0 if(n <= 1)

    if(n % 3 == 0 || n % 5 == 0)
      n + sum_multiples_3_and_5(n - 1)
    else
      sum_multiples_3_and_5(n - 1)
    end
  end

  def solve
    sum_multiples_3_and_5(999)
  end

  def correct?
    solve === answer
  end

  private

  def answer
    233168
  end
end

 

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