Bar modelling is becoming increasingly popular in schools as a way to represent an abstract calculation. In these pages the bar model approach can be used to help find the third number to make a total of 100. There are various ways children might approach this task. Some will add on from the total of the two numbers given. Others will subtract each number from 100 and see what is left. The best approach is the one which works quickest and most accurately and could vary from question to question. It is always worth asking children how they reach their answers as this shows a great deal about their understanding of addition.