If the missing number is an addend, students use the count on strategy or the relationship between addition and subtraction to find the missing number. And if the missing number is the sum of the addition sentence, they add the addends to get the sum. This worksheet, single digit addition within 5, provides plenty of practice with this concept. In each problem, the numbers are laid out in the vertical format. In this format, pair of digits in each successive place are tackled one by one. This helps students follow a structured approach.