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Column addition and subtraction

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Apuntes de la lección

What is Column Addition?

  • Column addition is a way to add numbers by writing them one under the other, lining up the digits by place value (ones, tens, hundreds, thousands).
  • Always start adding from the rightmost column (the ones column) and move left.
  • Carrying (also called regrouping) happens when a column adds up to 10 or more. You write the ones digit and carry the tens digit to the next column on the left.
  • Example: 27 + 59. Add 7 + 9 = 16. Write 6 in the ones column and carry 1 to the tens column. Then add 2 + 5 + 1 = 8. So 27 + 59 = 86.
  • You can add numbers with up to 4 digits using this method, like 3456 + 2789.

What is Column Subtraction?

  • Column subtraction is a way to subtract numbers by writing them one under the other, lining up the digits by place value.
  • Start from the rightmost column (ones) and move left.
  • Borrowing (also called exchanging) is needed when the top digit is smaller than the bottom digit in a column.
  • To borrow, you take 1 from the next column to the left, making it 10 in the current column. Then subtract.
  • Example: 47 - 19. In the ones column, 7 - 9 is too small, so borrow 1 from the tens. The 4 becomes 3, and the 7 becomes 17. 17 - 9 = 8. Then 3 - 1 = 2. So 47 - 19 = 28.
  • You can subtract numbers with up to 4 digits using this method, like 5000 - 2345.

Estimating Answers

  • Before you calculate, it is a good idea to estimate the answer. This helps you check if your final answer is sensible.
  • To estimate, round each number to the nearest ten, hundred, or thousand, then do the calculation with the rounded numbers.
  • For example, to estimate 3456 + 2789, round to the nearest thousand: 3000 + 3000 = 6000. The real answer is close to 6000.
  • For subtraction, estimate like this: 5000 - 23455000 - 2000 = 3000.
  • Estimating gives you a rough idea, not the exact answer.

Checking with the Inverse Operation

  • You can check your answer using the inverse operation. Addition and subtraction are opposites.
  • If you added two numbers, subtract one of them from your answer. You should get the other number.
  • Example: 27 + 59 = 86. Check: 86 - 59 = 27. ✔
  • If you subtracted, add the number you subtracted to your answer. You should get the starting number.
  • Example: 47 - 19 = 28. Check: 28 + 19 = 47. ✔
  • Always check your work to catch mistakes.

Choosing a Method

  • Sometimes you can add or subtract mentally (in your head) without writing columns.
  • Use mental math for easy numbers, like 20 + 30 or 100 - 40.
  • Use column method for bigger numbers or when you need to carry or borrow.
  • If you are not sure, you can always use the column method – it works for any numbers.
  • Practice both ways so you can choose the quickest.

Solving Word Problems

  • Word problems tell a story and ask a question. You need to find the numbers and decide what to do.
  • Look for key words that hint at addition: 'in all', 'total', 'together', 'more than'.
  • Look for key words that hint at subtraction: 'difference', 'left', 'fewer than', 'take away'.
  • Some problems need two steps: you might add first, then subtract, or vice versa.
  • Example: 'Sara has 125 stickers. She gets 80 more. Then she gives 45 away. How many does she have left?' First add 125 + 80 = 205, then subtract 45: 205 - 45 = 160.
  • Always read the problem carefully and underline the question.

Common Mistakes to Avoid

  • Always line up the digits by place value. If you don't, your answer will be wrong.
  • Don't forget to carry when a column adds to 10 or more.
  • Don't forget to borrow when the top digit is smaller than the bottom digit.
  • When borrowing, remember to reduce the digit you borrowed from by 1.
  • Check your work with estimation or the inverse operation.

Diapositivas

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Preguntas de práctica

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  1. 1.What is the digit that is transferred from one column to the next when adding numbers called?

    Easy
    • Acarry
    • Bborrow
    • Cregroup
    • Dexchange
  2. 2.In subtraction, the operation of taking a ten from the next column is called a borrow.

    Easy

    True or false?

  3. 3.Which of the following is the correct way to check that 456 + 389 = 845?

    Medium
    • ASubtract 389 from 845 to see if you get 456
    • BAdd 456 and 389 again
    • CMultiply 456 by 2
    • DSubtract 456 from 389
  4. 4.Which of the following sums require carrying (regrouping) when using the column method? (Select all that apply)

    Medium
    • A23 + 45
    • B56 + 37
    • C128 + 254
    • D345 + 210
    • E999 + 1
  5. 5.When adding 27 and 59, the carry from the ones column is 1.

    Easy

    True or false?

  6. 6.Match each calculation with the correct answer.

    Easy
    • 345 + 278
    • 504 - 267
    • 1234 + 5678
    • 623
    • 237
    • 6912
  7. 7.Put the steps for column addition of 27 and 59 in the correct order.

    Medium
    • Add the ones: 7+9=16, write 6 and carry 1
    • Add the tens: 1+2+5=8
    • Write the 8 in the tens column
    • Write the 6 in the ones column
  8. 8.Which of the following calculations require borrowing (exchanging) when using column subtraction? (Select all that apply)

    Medium
    • A52 - 18
    • B63 - 21
    • C400 - 156
    • D78 - 34

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