ABSTRACT
This is another essential skill in simplifying mental calculations – many of us separate the tens and the units when adding pairs of 2-digit numbers (and some do for subtracting). At the heart of this is an understanding that the result of adding the units in a pair of numbers is not affected by what happens with the tens: for example, 4 + 5 are 9, and 14 + 25 are 39 – the 9 remains consistent. This activity attempts to make this clear to children.
