In this short video clip Sarah is trying to solve 8 + ___ = 15.  The audio is a little difficult to hear but you can see that Sarah draws a number line and puts the number 8 and then the number 15 on it.  She takes a jump of 2 to get to ten and then she knows that she needs to take a jump of 5 to get to 15.  She adds the 2 and 5 together to get 7.