If an addend is missing, students use the count-on method or the relationship between addition and subtraction to determine the missing number. Also, they add the addends if the missing number is the sum of the addition sentence. This worksheet, add two multiples of 100, offers a tonne of practice with this idea. 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.