Add a one-digit number to a three-digit number: step-by-step example
30 October 2026
Adding a one-digit number to a three-digit number in your head is a useful skill for quick mental maths. Here is a clear, simple example to help your child see each step involved.
Worked example
345 + 7
First, look at the ones. 5 + 7 = 12.
12 is more than 10, so we need to regroup.
Keep the 2 in the ones place. Add the 1 (from 12) to the tens.
4 (tens) + 1 = 5 (tens).
So, the new number is 352.
The correct answer is 352.
Click or tap on the number line to answer
After seeing this worked example, your child will recognise how to handle regrouping when the ones add up to more than ten. They can now try similar mental addition questions to build confidence.
