Although column subtraction is specified for Year 3 in the National Curriculum, Hamilton believe that teaching column subtraction in Year 4 will be more successful. The National Curriculum Guidance specifies that teachers can teach areas of study in different year groups than those mentioned in the Curriculum, so long as they are taught within the same Key Stage. You can choose to teach this column subtraction content in Year 3 or in Year 4.
Day 1 Teaching
Display a word problem where we have to find how much further to cycle from 625 to 874 miles. Sketch a bar model to represent this then discuss how we can count up using Frog to find a difference. Model using Frog and adding the hops. Repeat this for 524 – 378 and 705 – 567.
Use the ‘Wonderful 1089’ in-depth problem-solving investigation below as today’s group activity.
Or, use this activity:
-- Use counting up to find the difference between two 3-digit numbers.
Day 2 Teaching
Write on the board 725 – 462. We can easily subtract the ones, but we have 20 subtract 60. What do we do? Discuss taking a hundred and adding it to the tens. This process is called ‘decomposition’. Why do you think this might be? Model expanded column subtraction. Repeat for 417 – 254 and 624 – 381.
--Use expanded column subtraction to subtract 3-digit numbers using decomposition.
Day 3 Teaching
Write 403 – 267. Work this out using expanded decomposition. Make sure every child follows the process. Point out that this was quite a performance! Say that we could, in this case, use Frog. Model counting up. Point out that counting up is often easier if there is a zero in the larger number.
-- Choose an efficient method to subtract pairs of 3-digit numbers.
-- Solve word problems, which involve subtracting 3-digit numbers, choosing an appropriate method.
Day 4 Teaching
Display four subtractions (see resources) and ask which are correct. Identify errors and discuss methods. Write 654 – 456 and discuss methods. Repeat for 753 – 357. Point out we are subtracting numbers with reversed digits.
-- Investigate patterns that occur when subtracting reversed 3-digit numbers.