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 add multiples of 100 and 3-digit numbers. 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.