Subtraction

Objectives

To apply different methods for place value subtraction and to establish which method is most comfortable.

Resources

Abacus 4 Textbooks 1 & 2 | Scanner or copier | Squared paper| Subtracting multiples of 10 from 4 digit numbers (YouTube)

Today's Lesson

Main Activity

Turn to page 85 in Abacus 4 Textbook 1 and ask your student to complete the multiplication charts mentally, as the warm up for today’s lesson.

Turn to page 11 in Abacus 4 Textbook 2.

Watch Subtracting multiples of 10 from 4 digit numbers (YouTube)

Look at the subtraction calculations with your student. Recognise that in previous sessions you were adding multiples of 10 or 100 by counting on in 10s or 100s OR by using number facts to help. Tell your student that today they will be doing the same thing, but this time subtracting multiples of 10 or 100 so they will need to count back in 10s or 100s, or use number facts.

Show for example 4119 - 50 and discuss whether or not this will cross a multiple of 100. Because we are

subtracting 50 and the tens number in our 4-digit number is 10, we will have to cross a hundred! So 4119 count back five 10s: 4109, 4099, 4089, 4079, 4069.

Repeat for 7381 – 600 = by counting back in 100s, taking care as you cross the hundreds boundary. Recognise that this subtraction could also be solved using number facts: 600 = 300 + 300 so you could have done 7381 – 300 = 7081, 7081 – 300 = 6781. Remind your student that they can subtract using number facts or count back to solve these. Students choose the method that suits them and gives them the right answer!

NB Students should not be using a vertical written method with ‘regrouping’ to solve ‘place value’ calculations such as these.

Citations

[1] www.youtube.com [2] www.youtube.com