title: Summing multiples of 3 and 5 under 1000 using Python
body: <p>I've written a program meant to find the sum of all multiples of 3 and 5 under 1000. The output I'm getting is <code>266333</code>, but that's not the correct answer. Please help, here is my code:</p>
  
  <pre><code>x = 3
  y = 5
  x_list = []
  y_list = []
  
  while x &lt; 1000:
      x_list.append(x)
      x += 3
  
  while y &lt; 1000:
      y_list.append(y)
      y += 5
  
  numsum = sum(x_list + y_list)
  print(numsum)
  </code></pre>