Year 2 Adding and subtracting: check-up and quiz

Key words. A fact is a sum that you know. Know the facts to 20 without counting.

Method. Use the way that makes the fact easy to hold in your head.

1. Start with the bigger number. 8 is the bigger number, so start there.
   8
2. Make 10. 8 needs 2 more to make 10.
   8 + 2 = 10
3. Add what is left. 2 of the 7 makes 10, so 5 is left.
   10 + 5 = 15

Check. Take 7 away from 15. You get 8. ✓

Watch out. Do not count from one again. Start at the bigger number.

Key words. Related facts use the same three numbers.

Method. Think of the adding fact that goes with it.

1. Look at the take-away. Take 9 away from 14.
   14 − 9
2. Think of the addition that matches. 9 + ? = 14 is the same fact.
   9 + 5 = 14
3. Write the answer. The missing part is 5.
   5

Check. 5 + 9 = 14. ✓

Watch out. Take away and add are linked. Use the adding fact to check.

Key words. A ten is a bundle of ten ones.

Method. The same fact works with tens. Each number is 10 times bigger, and so is the answer.

1. Look at the fact 3 + 6 = 9. This is the fact we know.
   3 + 6 = 9
2. Make each number 10 times bigger. 3 becomes 30 and 6 becomes 60.
   3 tens = 30, 6 tens = 60
3. Work out the answer. The answer is 9 tens, so it is 90.
   30 + 60 = 90

Check. 9 tens is 90. ✓

Watch out. Use the fact you know. Put a zero on each number and on the answer.

Key words. An exchange swaps 1 ten for 10 ones, or 10 ones for 1 ten.

Method. Work with the ones first. Then keep the tens.

1. Look at the ones. 7 + 6 = 13. That is 10 or more ones, so make a new ten.
   7 + 6 = 13
2. Make a new ten. 13 ones is 1 ten and 3 ones.
   13 = 1 ten and 3 ones
3. Add the new ten to the tens. 4 tens and 1 more ten make 5 tens.
   50
4. Put the parts together. 50 and 3 make 53.
   50 + 3 = 53

Check. Take 6 away from 53. You get 47. ✓

Watch out. Work with the ones first. If the ones do not fit, take a ten and make ten ones.

Key words. An exchange swaps 1 ten for 10 ones, or 10 ones for 1 ten.

Method. Work with the ones first. Then keep the tens.

1. Look at the ones. You cannot take 8 from 3, so take 1 ten.
   3 is less than 8
2. Exchange 1 ten for 10 ones. 6 tens becomes 5 tens and 13 ones.
   3 + 10 = 13
3. Take the ones away. 13 − 8 = 5
   13 − 8 = 5
4. Put the parts together. 50 and 5 make 55.
   50 + 5 = 55

Check. Add 8 to 55. You get 63. ✓

Watch out. Work with the ones first. If the ones do not fit, take a ten and make ten ones.

Key words. Adding tens means adding groups of ten.

Method. Only the tens change. The ones stay the same.

1. Look at the tens. 54 has 5 tens. We add 3 tens.
   5 tens + 3 tens
2. Work out the new tens. 5 + 3 = 8, so there are 8 tens.
   8 tens = 80
3. Keep the ones the same. The ones digit is 4 and it does not change.
   4 ones stay
4. Put the parts together. 80 and 4 make 84.
   80 + 4 = 84

Check. 54 + 30 = 84. Count in tens to check. ✓

Watch out. Do not change the ones digit. 54 becomes 84, not 55.

Key words. A written method splits the numbers into tens and ones.

Method. Split each number into tens and ones. Work with the tens, then the ones.

1. Add the ones. 7 + 5 = 12. That is 10 or more, so make a new ten.
   7 + 5 = 12
2. Write the ones and carry the ten. 12 ones is 1 ten and 2 ones. Write 2; carry 1 ten.
   12 = 1 ten and 2 ones
3. Add the tens, with the new ten. 4 tens + 3 tens + 1 ten = 8 tens.
   40 + 30 + 10 = 80
4. Put the parts together. 80 and 2 make 82.
   80 + 2 = 82

Check. Take 35 away from 82. You get 47. ✓

Watch out. Tens go with tens, and ones go with ones. Do not mix them up.

Key words. A written method splits the numbers into tens and ones.

Method. Split each number into tens and ones. Work with the tens, then the ones.

1. The ones are not enough, so exchange a ten. 2 ones is less than 8 ones, so take 1 ten.
   7 tens and 2 ones = 6 tens and 12 ones
2. Take away the ones. 12 ones − 8 ones = 4 ones.
   12 − 8 = 4
3. Take away the tens. 6 tens − 4 tens = 2 tens.
   60 − 40 = 20
4. Put the parts together. 20 and 4 make 24.
   20 + 4 = 24

Check. Add 48 to 24. You get 72. ✓

Watch out. Tens go with tens, and ones go with ones. Do not mix them up.

Method. Find two numbers that make 10 first. Then add the last number.

1. Look for two numbers that make 10. 5 and 5 make 10.
   5 + 5 = 10
2. Add the last number. Then add the number that is left.
   10 + 7 = 17
3. Write the answer.
   17

Check. Add them in a different order to check: 5 + 7 + 5 = 17. ✓

Watch out. Keep the numbers in your head. Do not forget the last number.

Method. The box is a number we do not know. Do the opposite to find it.

1. Read the sum with the box. The box stands for the missing number.
   ? + 16 = 39
2. Do the opposite to find it. To undo adding 16, take 16 away from 39.
   39 − 16 = 23
3. Check it in the sum. Put the number in the box and see if it works.
   23 + 16 = 39

Check. 23 + 16 = 39. ✓

Watch out. Adding undoes taking away. Taking away undoes adding.

Key words. A fact is a sum that you know. Know the facts to 20 without counting.

Method. Use the way that makes the fact easy to hold in your head.

1. Start with the bigger number. 9 is the bigger number, so start there.
   9
2. Make 10. 9 needs 1 more to make 10.
   9 + 1 = 10
3. Add what is left. 1 of the 6 makes 10, so 5 is left.
   10 + 5 = 15

Check. Take 6 away from 15. You get 9. ✓

Watch out. Do not count from one again. Start at the bigger number.

Key words. Related facts use the same three numbers.

Method. Think of the adding fact that goes with it.

1. Look at the take-away. Take 8 away from 17.
   17 − 8
2. Think of the addition that matches. 8 + ? = 17 is the same fact.
   8 + 9 = 17
3. Write the answer. The missing part is 9.
   9

Check. 9 + 8 = 17. ✓

Watch out. Take away and add are linked. Use the adding fact to check.

Key words. A ten is a bundle of ten ones.

Method. The same fact works with tens. Each number is 10 times bigger, and so is the answer.

1. Look at the fact 7 + 2 = 9. This is the fact we know.
   7 + 2 = 9
2. Make each number 10 times bigger. 7 becomes 70 and 2 becomes 20.
   7 tens = 70, 2 tens = 20
3. Work out the answer. The answer is 9 tens, so it is 90.
   70 + 20 = 90

Check. 9 tens is 90. ✓

Watch out. Use the fact you know. Put a zero on each number and on the answer.

Key words. An exchange swaps 1 ten for 10 ones, or 10 ones for 1 ten.

Method. Work with the ones first. Then keep the tens.

1. Look at the ones. 9 + 6 = 15. That is 10 or more ones, so make a new ten.
   9 + 6 = 15
2. Make a new ten. 15 ones is 1 ten and 5 ones.
   15 = 1 ten and 5 ones
3. Add the new ten to the tens. 5 tens and 1 more ten make 6 tens.
   60
4. Put the parts together. 60 and 5 make 65.
   60 + 5 = 65

Check. Take 6 away from 65. You get 59. ✓

Watch out. Work with the ones first. If the ones do not fit, take a ten and make ten ones.

Key words. Adding tens means adding groups of ten.

Method. Only the tens change. The ones stay the same.

1. Look at the tens. 86 has 8 tens. We take away 4 tens.
   8 tens − 4 tens
2. Work out the new tens. 8 − 4 = 4, so there are 4 tens.
   4 tens = 40
3. Keep the ones the same. The ones digit is 6 and it does not change.
   6 ones stay
4. Put the parts together. 40 and 6 make 46.
   40 + 6 = 46

Check. 86 − 40 = 46. Count in tens to check. ✓

Watch out. Do not change the ones digit. 86 becomes 46, not 85.

Key words. A written method splits the numbers into tens and ones.

Method. Split each number into tens and ones. Work with the tens, then the ones.

1. Add the ones. 8 + 6 = 14. That is 10 or more, so make a new ten.
   8 + 6 = 14
2. Write the ones and carry the ten. 14 ones is 1 ten and 4 ones. Write 4; carry 1 ten.
   14 = 1 ten and 4 ones
3. Add the tens, with the new ten. 3 tens + 4 tens + 1 ten = 8 tens.
   30 + 40 + 10 = 80
4. Put the parts together. 80 and 4 make 84.
   80 + 4 = 84

Check. Take 46 away from 84. You get 38. ✓

Watch out. Tens go with tens, and ones go with ones. Do not mix them up.

Key words. A written method splits the numbers into tens and ones.

Method. Split each number into tens and ones. Work with the tens, then the ones.

1. The ones are not enough, so exchange a ten. 1 ones is less than 6 ones, so take 1 ten.
   8 tens and 1 ones = 7 tens and 11 ones
2. Take away the ones. 11 ones − 6 ones = 5 ones.
   11 − 6 = 5
3. Take away the tens. 7 tens − 4 tens = 3 tens.
   70 − 40 = 30
4. Put the parts together. 30 and 5 make 35.
   30 + 5 = 35

Check. Add 46 to 35. You get 81. ✓

Watch out. Tens go with tens, and ones go with ones. Do not mix them up.

Method. Find two numbers that make 10 first. Then add the last number.

1. Look for two numbers that make 10. 2 and 8 make 10.
   2 + 8 = 10
2. Add the last number. Then add the number that is left.
   10 + 6 = 16
3. Write the answer.
   16

Check. Add them in a different order to check: 6 + 8 + 2 = 16. ✓

Watch out. Keep the numbers in your head. Do not forget the last number.

Method. The box is a number we do not know. Do the opposite to find it.

1. Read the sum with the box. The box stands for the missing number.
   ? − 15 = 40
2. Do the opposite to find it. To undo taking 15 away, add 15 to 40.
   40 + 15 = 55
3. Check it in the sum. Put the number in the box and see if it works.
   55 − 15 = 40

Check. 55 − 15 = 40. ✓

Watch out. Adding undoes taking away. Taking away undoes adding.

Method. Work out 53 + 29. Then compare it with what Sam said.

1. Work out 53 + 29. 3 + 9 = 12 ones, so make a new ten. 5 + 2 + 1 = 8 tens.
   53 + 29 = 82
2. Compare with Sam. Sam says 72, but the answer is 82.
   72 is not 82
3. Write the answer. Sam is wrong.
   No. 53 + 29 = 82.

Check. Take 29 away from 82. You get 53. ✓

Watch out. Carry the new ten into the tens column.