Here we have a set of worksheets which looks at addition of two 2-digit numbers, but they have been organised so that the tens boundary is not crossed. In other words, the total of the units is always less than ten. This is a good exercise as it highlights the possibility of carrying out a mental addition in two different ways. Usually (but not always) adults will carry out this type of sum by adding the tens first and then the units. This is directly opposite to how it is done on paper, but somehow the numbers seem easier to work with this way round. Children may well start with the units and then add the tens. It does not matter which approach they take as long as they become confident and quick with the chosen method. Adding tens and units without crossing the tens boundary