Students will learn how to add certain three-digit numbers mentally. For example, to do 355 + 480, I can take 20 from 355 and tack it on to 480. Now I'm doing 335 + 500, which is much easier. Likewise, to do 18 + 704, I remove 4 from 704, and tack it on to 18. Now I'm doing 22 + 700, which is again, much easier. Students could also think of 18 + 704 as 12 + 710, which I think is slightly more difficult, but they can do whatever they prefer. Here's a lesson.