Add the first problem from project Euler

Project Euler/Problem 1/problem1_sol1.rb

@@ -0,0 +1,14 @@
+# 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 divisible_by_three_or_five?(number)
+  (number % 3).zero? || (number % 5).zero?
+end
+
+sum = 0
+(1...1000).each do |i|
+  sum += i if divisible_by_three_or_five?(i)
+end
+
+p sum