What is a sum?

We explain what a sum is and how children are taught to understand the term when it appears in mental maths tests, word problems or investigations.

What is a sum?

The sum of two numbers is the answer you get when you add them both together. So the sum of 5 and 4 is 9.

There was a time when teachers used the word 'sum' to name the following addition number sentence:

9 + 5 = 14

They would often then mistakenly use the same word to name these subtraction, multiplication and division number sentences:

9 - 4 = 5
7 x 3 = 21
80 ÷ 10 = 8

To get round this confusion, we now refer to all of the above (in bold) as number sentences.

Sums in KS1 and KS2 maths

Children in Key Stage 2 need to be familiar with the word 'sum' as it often comes up in mental maths tests. For example:

What is the sum of 6 and 12?

What is the sum of 8, 2 and 9?

They may also carry out word problems or investigations in maths lessons which rely on them being familiar with the word, for example:

Word problem:

Mary has £35 in one bank account and £82 in another. What is the sum of money in both her bank accounts?
Word problem answer: £117


I am thinking of a two-digit number. The sum of both its digits is 9. The number is bigger than 50 and a multiple of 9. What could it be?
Investigation answer: 54, 63, 72, 81 or 90

Children need to be exposed to a variety of mathematical vocabulary as often as possible, rather than just seeing numbers and symbols.