Subtracting from a 2 digit number
Show the number square (below) and explain that, if we need it, we will use this to help us subtract. We will be subtracting 2-digit numbers by counting back in 10s and 1s.
Write down 76 – 35 = and read it together.
Model solving the subtraction by counting back three 10s from 76 to 46, then subtracting the 5. Most children should know 6 – 5 = 1 because 1 + 5 = 6. So 76 – 35 = 41.
Demonstrate this using Number square tool.
Repeat, calculating 58 – 23 =. Remind them to count back the 10s then subtract the 1s by using the number facts 8 – 3, or by counting back three 1s.
When you have done this, model using the number square. Demonstrate counting back 20 (up two 10s on the grid) to 38, then back 3 to 35.
Remind children that subtraction cannot be done in any order; we canʼt swap the numbers round and still get the same answer. Model answering 56 – 43 = by counting back four 10s then three 1s. Then model switching it round to 43 – 56 = and ask children why this does not give the same answer. Show nine objects and ask children to take away six. How many are left? Now ask them to try and take nine objects away from six and see what happens! Agree that there aren't enough objects to subtract nine.
Adding 2 digit numbers
Explain to children they will be adding two 2-digit numbers. We are all getting really good at this so it should be no problem! Write on the whiteboard 26 + 42 =. Read it together.
Model starting with the larger number, 42, rewriting the addition if necessary: 42 + 26 =. Then demonstrate adding 26 by adding 20 (62), then adding on 6. Use the Number square tool to model the addition. Children should know 6 + 2 = 8 but they can count on in 1s if absolutely necessary.
Write 53 + 45 = and 66 + 27 =. Ask children to work with a partner to solve 53 + 45 and 66 + 27.
For 66 + 27, check that they first added 20 to 66 to give 86. Then discuss how they added 7 to 86. Some children will have worked out 86 + 7 by bridging 10 (adding 4, to give 90, then adding 3).
How can you add these two numbers?
Which number will you begin with? Why?
What do you do first? Then what?
Which number facts can help us solve this?
Why is it better to use number facts instead of counting on?Will the answer be more or less than 100? How do you know?
Display the tuck shops (below) Visit the shop and choose two things, e.g. the apple and the banana. Write the prices as an addition, i.e. 31p + 26p =.
Use 10p coins and 1p coins to make each price. How could we find the total cost of buying these two things? Suggest that we count the 10ps and then the 1ps. How many 10ps do we have altogether? And 1ps? Complete the addition: 50p + 7p = 57p.
Repeat, this time making sure that items chosen have 1s digits with a total of more than 10p (e.g. 38p + 47p).
How can we add 70p and 15p? That’s easy: add 15 by adding 10 then 5! So the total is 85p.
How many 10ps and how many 1ps are in that price?
How many 10ps do you have altogether? How many 1ps? What is the total money you have spent?
Can you find the total without using coins? How will you do it?
Move on to buying three items at a time, with a total of more than £1. Use 10p and 1p coins to help if needed. We have 130p in 10p coins. How many 10p coins make £1? So we have £1 and 30p in 10p coins. Now, how many 1p coins do we have? So, how much do we have altogether? Whatʼs the total of our 10p and 1p coins?