Here is a revision page on adding two teen numbers mentally and is probably best suited to year 3 children but could also be very useful for older children who are not confident with adding 2-digit numbers.
As I have said before, it is interesting that when we add ‘in our heads’ we often do it in a very different way than if we were using pencil and paper methods. For example 15 + 16 can be done several ways, non of which is significantly better or worse than another.
method 1: add the two tens to make 20. Add 5 to make 25 and then add (or count on) 6 to make 31.
method 2: add 10 to 16 to make 26 and then add 5 to make 31.
method 3: add 10 to 15 to make 25 and then add 6 to make 31.
method 4: recognise that 15 + 16 is nearly double 15 which is 30 and then make an adjustment of 1 to make 31.
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