Using Landmark Numbers on an Open Number Line

A landmark number is simply a number that is easy to work with. In second grade terms, the easiest numbers to work with are numbers we use when we "skip count by tens" (10, 20, 30). In using landmark numbers on the open number line, we want to find a landmark number that is easy to count on from.  There are many variations to this strategy. The example below shows one of the most common variations. 

Example: 28 + 17 

Students will think28 plus something is going to get me to my next landmark number of 30.

I know that 28 plus 2 will get me to the landmark 30. 

Since I am adding 17 altogether, I need to subtract 17-2 (the hop I already took) to  equal 15. This tells me I still need to take a hop of 15 on the number line. If students cannot add 30 + 15 in their head, they can break apart the 15 in a way that is helpful to them. (10 + 5 hops, 15 single hops, etc.)