Students use the count-on method or the relationship between addition and subtraction to find the missing number if it is an addend. Also, they add the addends to get the sum if the missing number is the sum. There is a lot of practice with this concept in the worksheet single digit addition within 5. In each problem, the numbers are laid out in the horizontal format. Students should try to use different strategies involving composing and decomposing numbers to solve these problems. This will help them develop flexibility and fluency.