Written subtraction within 100

What you will learn

• Exchange one ten for ten ones and complete written subtraction within 100

Models used: place-value charts, number sentences, bar models, stories.

Before you start

• Add and subtract within 100

Exchange, then subtract

Work out 52 − 28.

1. Line up ones under ones and tens under tens. Start on the right.

   52 − 28

   	tens	ones
   First number	5	2
   −	2	8
   Answer	?	?

2. Exchange 1 ten for 10 ones. The top number still has the same value.

   1 ten = 10 ones

   	tens	ones
   First number	54	212
   −	2	8
   Answer	?	?

   1 ten⇄10 ones

3. Now subtract the ones. Write 4 in this column.

   12 − 8 = 4

   	tens	ones
   First number	54	212
   −	2	8
   Answer	?	4

4. Now subtract the tens. Write 2 in this column.

   4 − 2 = 2

   	tens	ones
   First number	54	212
   −	2	8
   Answer	2	4

5. Read the answer from left to right. Add back the amount taken away to check.

   24 + 28 = 52

   	tens	ones
   First number	54	212
   −	2	8
   Answer	2	4

Worked example: Fill one answer box at a time

Work out 71 − 36.

1. Line up ones under ones and tens under tens. Start on the right.

   71 − 36

   	tens	ones
   First number	7	1
   −	3	6
   Answer	?	?

2. Exchange 1 ten for 10 ones. The top number still has the same value.

   1 ten = 10 ones

   	tens	ones
   First number	76	111
   −	3	6
   Answer	?	?

   1 ten⇄10 ones

3. Work in the highlighted column.

   	tens	ones
   First number	76	111
   −	3	6
   Answer	?	?

   What digit belongs in the ones column?

   5

4. Work in the highlighted column.

   	tens	ones
   First number	76	111
   −	3	6
   Answer	?	5

   What digit belongs in the tens column?

   3

5. Read the answer from left to right. Add back the amount taken away to check.

   35 + 36 = 71

   	tens	ones
   First number	76	111
   −	3	6
   Answer	3	5

Common mistakes to discuss

• Swap subtraction digits. Keep the order. Exchange from the next place when the top digit is too small.
• Change only one column. Each exchange reduces the next place by one and adds ten to the place beside it.

Check your understanding

• Keep the starting value unchanged when exchanging.
• Exchange one ten for ten ones.
• Check the difference with addition.

Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.