project_euler_1.rb

# Multiples of 3 and 5
# 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.

puts (1...1000).select { |n| n % 3 == 0 || n % 5 == 0 }.reduce(:+)