1. Mental Subtraction – Objectives
Subtract three-digit numbers from multiples of 100 by counting up.
You may need:
Mini whiteboards
© Hamilton Trust Stepping Up Term Week 8 Day 1

2. Mental Subtraction – Direct Teaching
Remember how you can count up using knowledge of pairs with a total of 10 or 100 to work out subtractions such as:
= 33
167
200
170 3 30
>
Briefly remind students of this strategy to find a difference from ‘Week 3’.
© Hamilton Trust Stepping Up Term Week 8 Day 1

3. Mental Subtraction – Direct Teaching
Or…
= 42
358
400
360 2 40
>
© Hamilton Trust Stepping Up Term Week 8 Day 1

4. Mental Subtraction – Student Activity How many are needed to make the
next multiple of 100?
Use pairs to 10 and then think what multiple of 10 needs to be added to reach the 100s number…
+11
+51
+65
+37
+29
+25
+52
+46
+15
+62
+75
+58
289
649
135
763
871
375
548
454
685
538
725
842
Students work with a partner and record answers on their whiteboards. Use animation to reveal answers.
© Hamilton Trust Stepping Up Term Week 8 Day 1

5. Mental Subtraction – Direct Teaching
We can use the same counting up strategy
to work out subtractions such as:
211
700 – 489 = 1
490 10
500
200
Students briefly discuss, then feedback. Model ‘counting up’ in 3 jumps to find the difference: 1s, 10s then 100s.
489
700
© Hamilton Trust Stepping Up Term Week 8 Day 1

6. Mental Subtraction – Direct Teaching
You might record the first two jumps as one jump, but the answer will still be the same.
222 = 22
400
200
378
600
© Hamilton Trust Stepping Up Term Week 8 Day 1

7. © Hamilton Trust Stepping Up Term 1 Week 8 Day 1
Mental Subtraction – Individual Practice
Estimate which of these will have the greatest and least answers, then work them out…
Challenge
Can you write 5 more subtraction questions like this - with an answer of 353?
400 – 267
900 – 583
700 – 456
800 – 429
600 – 444
700 – 562
Students work in the back of their books or on mini-whiteboards, completing list of questions before the challenge activity.
Watch out for students who:
• Think that 47 needs to be added to 63 to make 100, because they look for tens which add to 100, not 90;
• Try to keep all the steps in their head and lose track, encourage them to use jottings, modelling how to use these if necessary.
© Hamilton Trust Stepping Up Term Week 8 Day 1