This method counts all the numbers that are multiples of the numbers you give.

multiplier.rb

# 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.

def multiples(number, *multipliers)
  count = 0
  list  = []

  # This range excludes the number you give
  # Natural numbers starts at 1
  (1...number).each do |num|
    multipliers.each do |mult|
      next unless num % mult == 0
         
      unless list.include?(num)
        count += num
        list << num
      end
    end
  end
  count
end

Benchmark

require 'benchmark'
Benchmark.bmbm { |b| b.report { multiples(1000, 3,5)} }