Adding three or more numbers
When faced with three numbers to add together, a pupil may carry out two separate calculations - adding two numbers first, then adding the total to the third number. It may be necessary to demonstrate how the same answer can be achieved by adding the three numbers together in a single calculation.
When adding two numbers together using column addition, a carried digit can only ever be 1. However, when adding more than two numbers, the carried digit can be a 1, 2 or higher. Some pupils may continue to always carry a 1 though. They need to understand that when adding up the units, the tens digit in the total needs to be the same as the digit they carry.
Other errors can be due to insecurity with place value, particularly with reading and writing ‘teen’ numbers.
and record as
Some pupils write teen numbers in the order they hear them – when hearing 'four teen' they write the 4 first, then the 1 in front of it. So in written addition, it will be easy to continue to write the first thing they hear (the 4 ) in the current column before carrying the 1 for the teen.
and record incorrectly as
Thus when having a total of 'twenty four' , they may incorrectly write the first thing they hear (the 2 ) in the current column, then carry the 4 .
Such pupils may need further practice of reading and writing 2-digit numbers, particularly 'teen' numbers, to build confidence and secure understanding, as well as needing reinforcement of how place value is maintained when carrying.
Strategies for adding 3 or more numbers mentally