According to the 3rd Grade Common Core Standard, 3.NBT.1, students need to learn to "Use place value understanding to round whole numbers to the nearest 10 or 100."
This is can be a difficult concept for some students when using a straight number line.
When students use these mountain-shaped number lines, students can understand easier the concept of being closer to the lesser or the greater number.
Using different colors for the numbers also helps the students to identify the correct answer.
I have attached one page explaining how to use this resource.
I hope you find it as useful as it is for me!