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addition

Adding two-digit numbers mentally

Split into tens and ones for flexible mental addition.

Quick answer

You can add two-digit numbers mentally by splitting them into tens and ones, then adding the tens together and the ones together.

Learning outcome

By the end of this page, you will be able to add two-digit numbers in your head by splitting them into tens and ones. Success signs:
  • You can split a two-digit number into tens and ones
  • You can add tens and ones separately
  • You can combine the answers to get the total
  • You can check your answer with a different method

Why this matters

Mental addition is a superpower! It helps you solve maths problems quickly without needing paper or a calculator. You use this skill when:
  • Adding up your pocket money
  • Calculating how many goals you scored in football
  • Working out how many cookies you need for a party
  • Figuring out how much change you should get
  • Estimating how long a journey will take
This skill builds a strong foundation for bigger maths problems and makes you feel confident with numbers.

How it works

Here's how to add two-digit numbers mentally:
  1. Split each number into tens and ones - Look at each number and see how many tens and ones it has
  2. Add the tens together - Put all the tens in one group and add them up
  3. Add the ones together - Put all the ones in another group and add them up
  4. Add the two totals together - Combine your tens total and ones total

Examples you can see

Let's try adding 24 + 35 using this method: Step 1: Split into tens and ones
  • 24 = 20 + 4
  • 35 = 30 + 5
Step 2: Add the tens
  • 20 + 30 = 50
Step 3: Add the ones
  • 4 + 5 = 9
Step 4: Add the totals
  • 50 + 9 = 59
Answer: 24 + 35 = 59 Let's try another one: 47 + 26 Step 1: Split into tens and ones
  • 47 = 40 + 7
  • 26 = 20 + 6
Step 2: Add the tens
  • 40 + 20 = 60
Step 3: Add the ones
  • 7 + 6 = 13
Step 4: Add the totals
  • 60 + 13 = 73
Answer: 47 + 26 = 73

Try it as a mini adventure

The Great Toy Store Adventure You're going to the toy store with £50 to spend. You see these toys:
  • Robot: £23
  • Car: £18
  • Ball: £12
  • Puzzle: £31
Challenge 1: Which two toys can you buy with your £50? Try adding the prices mentally using the split method:
  • Robot (£23) + Car (£18)
  • Robot (£23) + Ball (£12)
  • Robot (£23) + Puzzle (£31)
  • Car (£18) + Ball (£12)
  • Car (£18) + Puzzle (£31)
  • Ball (£12) + Puzzle (£31)
Challenge 2: What's the total cost of the robot and puzzle? Try: 23 + 31 Challenge 3: How much money do you have left after buying the robot and car? Try: 50 - (23 + 18)

Common mistakes

  • Mistake: Forgetting to regroup when ones add up to 10 or more
    • Why it happens: Children focus only on the ones place and forget that 10 ones make a ten
    • Fix: When ones add up to 10 or more, make a new ten and count the remaining ones
  • Mistake: Adding tens and ones separately but forgetting to combine them
    • Why it happens: Children get excited about the separate steps and forget the final step
    • Fix: Always remember to add your tens total and ones total together
  • Mistake: Mixing up which numbers are tens and which are ones
    • Why it happens: Confusion about place value
    • Fix: Use your fingers or draw lines to separate tens from ones

Parent and carer guide

Here's how you can help your child practice mental addition: 5-minute practice session:
  1. Start with simple numbers like 12 + 15
  2. Ask your child to split each number into tens and ones
  3. Have them add the tens first, then the ones
  4. Then add the two totals together
  5. Check the answer using a different method
What to say:
  • "Let's break this down - what are the tens in each number?"
  • "What happens when you add the tens together?"
  • "Now let's add the ones"
  • "Can you check your answer another way?"
What not to worry about:
  • It's normal for children to make mistakes at first
  • They might take longer than you expect
  • They may need to write down the steps at first
  • Keep it fun and encouraging
Real-life practice:
  • While shopping, ask "How much are these two items together?"
  • When cooking, ask "If we need 24 eggs and we already have 17, how many more do we need?"
  • At the park, "If there are 32 children on the swings and 19 on the slide, how many children are there altogether?"

Practice questions

  1. What is 24 + 35?
  2. What is 47 + 26?
  3. What is 58 + 13?
  4. What is 39 + 28?
  5. What is 64 + 17?
  6. What is 21 + 49?
  7. What is 73 + 16?
  8. What is 34 + 27?
  9. What is 52 + 38?
  10. What is 46 + 29?
Answers:
  1. 59
  2. 73
  3. 71
  4. 67
  5. 81
  6. 70
  7. 89
  8. 61
  9. 90
  10. 75

FAQ

What is mental addition?

Mental addition is adding numbers in your head without using paper or a calculator. It uses your brain to do the maths.

Why should I learn mental addition?

Mental addition helps you solve problems quickly, makes you more confident with numbers, and is useful in everyday life like shopping or playing games.

How can I get better at mental addition?

Practice splitting numbers into tens and ones, use the steps we learned, and try different problems. The more you practice, the faster and easier it gets.

What if I get confused with the ones that make ten?

When the ones add up to 10 or more, you make a new ten and count the remaining ones. For example, 7 + 6 = 13, so you have 1 ten and 3 ones.

Can I use this method for bigger numbers?

Yes! This method works for any numbers. You can split larger numbers into hundreds, tens, and ones, then add each part separately.