Number - Addition and subtraction
National Curriculum objective, Year 3, Addition and subtraction • add and subtract numbers mentally, including:
Adding and subtracting mentally Challenge 1:
three-digit numbers and tens
–
three-digit numbers and hundreds
b) 365 + 19 = 365 + 20 – 1 = 384
Accept answers that fit with what Fahmida has done.
c)
a)
234 + 16 = 230 + 10 = 250 using number bonds to 10
d) 345 + 18 = 350 + 13 = 363
b)
365 + 19 = 365 + 20 – 1 = 384 rounding and adjusting
e)
c)
444 + 21 = 444 + 20 + 1 = 465 sequencing
d)
345 + 18 = 350 + 13 = 363 bridging 10
e)
150 + 151 = 150 × 2 + 1 = 301 near doubles
444 + 21 = 444 + 20 + 1 = 465
150 + 151 = 150 × 2 + 1 = 301
Explain your reasoning in writing.
Assessment
Tom worked out the answers to some calculations.
Assess the explanations that pupils have written. Do they match with the way Fahmida has broken down her calculations? These are well known mental calculation strategies that pupils need to be taught.
He made some mistakes. a)
456 + 30 = 756
b)
614 + 7 = 611
d) 555 – 10 = 455
e)
150 – 9 = 159
c)
Note
789 + 20 = 7109
In the National Curriculum, pupils are required to perform the four operations mentally. There is no guidance on how. The mental calculation strategies mentioned above are useful ones. It is important to teach these so that pupils can decide on efficient methods to add and subtract. For every calculation, pupils need to think - can I do this in my head, use jottings or would a written method be more efficient? Once written methods are taught many pupils rely on using these even when they are not necessary. We need pupils to think flexibly. It might be a good idea to spend a week working on mental calculation strategies before teaching written methods.
Explain your reasoning in writing.
Addition and subtraction
Explain what he could have done wrong.
Challenge 2 Answer
I can use the column method to answer 256 + 240. That is the only way I know.
There are other methods that Tom can use. How many can you think of? Try to think of two other ways. Write explanations of what you could do.
Challenge 4:
–
Challenge 1 Answer
234 + 16 = 230 + 10 = 250
What mental calculation strategies has she used?
Challenge 3:
three-digit numbers and ones
Fahmida has added these numbers using different mental calculation strategies. a)
Challenge 2:
–
Solve these: a)
24?
b)
5?4
c)
7?3
d)
6?7
e)
457
f)
3?6
+ 3?6
+ ?76
+ ?59
+ 27?
+ ???
+ 26?
631
940
93?
?09
786
?53
All Tom’s answers were incorrect. a)
456 + 30 = 756: he added 300 instead of 30
b)
614 + 7 = 611: he added 7 to 4 to give 11, but he didn’t exchange the ten ones for a 10
c)
789 + 20 = 7109: he added 20 to 80 to make 100 but didn’t exchange the ten ones for 100 and add that to the 700
d)
555 – 10 = 455: he took away 100 instead of 10
e)
150 – 9 = 159: he added
Assessment Assess whether pupils can identify each error and if their explanations clearly show what Tom has done wrong. Ask pupils to show what the answers should have been.
Explain how you managed to find the missing numbers. Now you know all the numbers, find the answers to these addition calculations using a different method.
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National Curriculum objective, Year 3, Addition and subtraction
Number - Addition and subtraction
• add and subtract numbers with up to three digits, using formal written methods of columnar addition and subtraction
Challenge 3 Answer Tom could have used, for example, sequencing or doubling, as alternative methods. Any pupil’s method for a mental calculation strategy that has been explained and gives the correct answer is acceptable.
Addition and subtraction Challenge 5:
Assessment Assess whether pupils can use a variety of mental calculation strategies to answer this. Discuss with them whether they think the column written method is efficient. If they think it is, a focus on mental calculation is probably necessary in your whole class teaching. Did pupils think of sequencing (256 + 200 + 40 = 496)? What about doubling (240 + 240 + 16 = 496)?
a)
245 + 386 631
b)
564 + 376 940
c)
773 + 159 932
d)
637 + 272 909
e)
457 + 329 786
f)
There are other methods that Tom can use. How many can you think of? Try to think of two other ways. Write explanations of what you could do.
Challenge 6:
Challenge 4 Answer
I can use the column method to answer 548 – 299. That is the only way I know.
Solve these: a)
386 + 267 653
Assessment
456
b) ?73
c)
5?2
d)
835
e)
64?
f)
?60
– 23?
– 4?1
– ?64
– 4??
– 2?9
– 19?
?19
19?
26?
?61
?52
2?2
Explain how you managed to find the missing numbers.
Assess whether pupils can identify the missing digits. Discuss with them how they would go about doing this. Expect them to take each column in turn beginning with the ones. They should be able to work out the missing ones number quite easily. Most of the calculations involve at least one exchange. Do they remember to adjust the missing digits in the tens and hundreds positions correctly? What other methods do they use to calculate the additions? Look out for mental calculation strategies, such as sequencing or bridging ten. If pupils have simply used commutativity, ask them to find a third way. If they have subtracted, so used inverse, remind them that this could be used as a check but they need to add the original augend and addend.
Now you know all the numbers, find the answers to these subtraction calculations using a different method.
Estimating
Note Bridging ten is a great mental calculation strategy which should be taught to the whole class with simpler numbers initially. For example, 245 + 386, take 4 from 245 and add to 386 to make the calculation 241 + 390, then take 10 from 241 and add to 390 to make the calculation 231 + 400. This is now very simple to add.
Challenge 7:
486 + 398 =
I think the answer to this calculation is about 700. That’s my estimate!
Explain how Fahmida has made her estimate. Do you agree with her? If not, what would your estimate be? Write an explanation of your thinking.
Challenge 8:
Estimate the answers to these calculations. a)
215 + 398
b) 399 – 275
c)
675 + 289
d) 602 – 278
Now find the actual answers. How close were your estimates? 4
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National Curriculum objective, Year 3, Addition and subtraction
National Curriculum objective, Year 3, Addition and subtraction
• add and subtract numbers with up to three digits, using formal written methods of columnar addition and subtraction
Challenge 7 Answer
Challenge 5 Answer Tom could have used, for example, rounding and adjusting or counting on. He could have simply made the calculation simpler. Any method that has been explained by the pupils and gives the correct answer is acceptable.
Assessment Discuss with them whether they think the column written method is efficient for these particular numbers. If they think it is, a focus on mental calculation is probably necessary in your whole class teaching. Assess the calculation strategies that the pupils used. Were these carried out efficiently, fluently and accurately? Did pupils think of rounding and adjusting (548 – 300 + 1 = 249)? What about counting on (299 + 1 + 248)? Did any make the calculation simpler by adding one to each number to make 549 – 300? It might be worth working on this strategy with your class. If adding or subtracting the same number to the minuend and subtrahend the difference will always be the same.
Challenge 6 Answer: a)
456 – 237 219
b)
673 – 481 192
• estimate the answer to a calculation
Fahmida’s estimate is a little low. She has added the hundreds numbers together for her estimate. A closer estimate would be given if the numbers are rounded to the nearest 100: 500 and 400 to give an estimate of 900.
Assessment Assess pupils’ ability to round numbers to the nearest 100 to give an estimate of 900. If they cannot round or do so without confidence, you may need to work on this during whole class mathematics lessons. Did any pupils make a closer estimate by rounding to the nearest 10? 486 + 398 could be rounded to 490 and 400 to give an estimate of 880. Pupils giving this estimate would be showing a deeper level of mastery.
Note Rounding is an important skill. It is useful for finding approximate totals or differences which we often do in real life with money and also for the mental calculation strategy of rounding and adjusting.
Challenge 8 Answer c)
532 – 264 268
d)
835 – 474 361
e)
641 – 289 352
f)
460 – 198 262
Assessment Assess whether pupils can identify the missing digits. Discuss with them how they would go about doing this. Expect them to take each column in turn beginning with the ones. They should be able to work out the missing ones number quite easily. Most of the calculations involve at least one exchange. Do they remember to adjust the missing digits in the tens and hundreds positions correctly to allow for the exchanges? What other methods do they use to calculate the subtractions? Look out for mental calculation strategies, such as sequencing or counting on. Did any pupils make the calculation simpler, for example, adding 19 to 673 and 481 to make the calculation 692 – 500? Discuss with them how efficient these strategies are for subtraction calculations like these. They might be for some, but for others it might be more efficient to use the written method.
Note Making a subtraction simpler by adding or subtracting the same number to the minuend and subtrahend is a great strategy. We are finding the difference between the two, therefore, if we add or subtract the same number we are changing the values but not the difference. For example, 532 – 264, if we add 36 to both numbers we make the calculation 568 – 300. Of course, this relies on pupils being fluent with number bonds to 10, 20 and 100. This might be an area that needs to be focussed on if they are not.
To make their estimates pupils should have rounded to the nearest 100 or 10. a)
215 + 398: 200 + 400 = 600 or 220 + 400 = 620 Actual answer: 613
b)
399 – 275: 400 – 300 = 100 or 400 – 280 = 120 Actual answer: 124
c)
675 + 289: 700 + 300 = 1000 or 680 + 290 = 970 Actual answer: 964
d)
602 – 278: 600 – 300 = 300 or 600 – 280 = 320 Actual answer: 324
Assessment Assess whether pupils can estimate by rounding to the nearest 10, this shows a deeper understanding than rounding to 100. If they have simply rounded to 100, ask them if they can make a more accurate estimate. Assess how pupils found the actual answers. Did they think carefully about whether they could use a mental calculation strategy or whether it would be more efficient to use a written method? For example, 399 – 275 could be answered by sequencing: 399 – 200 – 70 – 5. A written method is not as efficient.
Note Ensure that you give pupils opportunities to rehearse and use mental calculation strategies because these are very important. Written methods are not always the most efficient. You might need to consider the calculations that you give to pupils to answer and allow them to decide which methods are most appropriate.
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