Stage 6 Subtraction

Strategy 1: Expanded

Question: 249 - 125

• Expand the numbers to look like this 200 + 40 + 9 100 + 20 + 5
• Subtract the ones 9 - 5 = 4
• Subtract the tens 40 - 20 = 20
• Subtract the hundreds 200 - 100 = 100
• Put them all together 100 + 20 + 4
• The answer is 124

Strategy 2: Decomposition

Question: 953 - 124

• Expand the numbers so they look like this 900 + 40 + 13 100 + 20 + 4 I did this because there are not enough ones to answer the question so I had to borrow ten from 50. So 50 became 40 and 3 became 13.
• Subtract the ones 13 - 4 = 9
• Subtract the tens 40 - 20 = 20
• Subtract the hundreds 900 - 100 = 800
• Put it all together 800 + 20 + 9
• The answer is 829

Strategy 3: Rounding & Compensating

Question: 246 - 98

• Decide which number to round to a tidy number. I will round 98 to 100 by adding 2.
• I have to add or subtract the same number to both sides. This is compensating. So now I have to add 2 to 246. 246 + 2 = 248
• The new question is 248 - 100
• The answer is 148

Strategy 4: Algorithm

Question: 672 - 311

• Stack the numbers so they look like this 672 311
• Subtract the ones. 2 - 1 = 1
• Subtract the tens. 7 - 1 = 6
• Subtract the hundreds 6 - 3 = 3
• 672 311 -------- 361 is the answer.

Strategy 5: Algorithm with Decomposition

Question: 672 - 328

• Stack the numbers.
• Subtract the ones. There are not enough ones so I have to borrow 10. Now I can do 12 - 8 = 4
• Subtract the tens. Because I borrowed 10 I had to change the 7 to a 6. Now I can do 6 - 2 = 4
• Subtract the hundreds 6 - 3 = 3
• 672 328 -------- 344 is the answer.

Strategy 6: Reversibility

Question: 68 + _ = 93

• If you don't want to add-on numbers to answer this question you can change or reverse it into a subtraction question.
• You need to move 68 to the other side of the = sign. When you do this it will become - 68
• The new question will look like this: 93 - 68 = ?
• Subtract the ones. Because you can't do 3-8 you have to borrow from the tens and do 13 - 8 = 5
• Subtract the tens. Because I borrowed 10 now I have to make it 8 - 6 = 2
• The answer is 25