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Project Euler: Problem 1 in Ruby

I've been working through some of the Project Euler problems in Ruby and I thought I'd post my solutions. I refuse to "cheat", but it's really neat to see all the different ways other people solve the problems afterwards.

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.

total = 0
 
1000.times do |i|
  if i % 3 == 0 || i % 5 == 0
    total += i
  end
end
 
puts total

I googled around a bit after I finished and particularly liked this solution, which seemed more ruby-ish to me:

answer = (0..999).select { |a| a%3 ==0 || a%5==0 }
puts answer.inject { |sum, n| sum+n }

And, from the forums I thought Olathe's solution was really neat, although mathily over my head:

class Integer
  def sum_mod modulus
    n = self.div modulus
    modulus * n * (n + 1) / 2
  end
end
 
num = 999
sum = num.sum_mod(3) + num.sum_mod(5) - num.sum_mod(15)
 
puts "Sum is #{ sum }."