Common Core homework. Since when is 500 reasonable when 645 is the answer? I don't see any reason to teach 'front end estimation'. If they're not ready for rounding in 3rd grade, skip this estimation nonsense and teach them how to round the right way when they're ready.
You have the power to ensure that your child gets what he or she needs by investing in who that child is, deeply, now; valuing all forms of communication; seizing opportunities to create real contexts for writing; dialoguing about language in its multifaceted natural contexts you encounter daily; and helping your child get his or her voice to the page or screen as often as is naturally possible. Read more: http://blog.bravewriter.com/2013/10/31/you-can-trust-it/