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, addition facts within 20, offers a tonne of practice with this idea. In this worksheet, students practice solving problems written in the horizontal format. How numbers are laid out in a problem affects the method a student employs to solve it. Therefore to develop actual fluency and mastery of multiple strategies, students must practice different formats.