Column method to 100

When the numbers feel too big to do in your head, use the column method. It always helps.

What it is

You write the two numbers one above the other so that the same places line up vertically:

• Ones under ones.
• Tens under tens.

Underneath you draw a line and you compute vertically: ones first, then tens.

Adding example: 34 + 25

Set them up:

   3 4
 + 2 5
 -----

Step 1 — ones. 4 + 5 = 9. Write 9 in the ones column. Step 2 — tens. 3 + 2 = 5. Write 5 in the tens column.

   3 4
 + 2 5
 -----
   5 9

Answer: 59. 34 + 25 = 59.

Example with regrouping: 47 + 28

   4 7
 + 2 8
 -----

Step 1 — ones. 7 + 8 = 15. Watch — that's more than 9.

• Write 5 in the ones column.
• Write the 1 extra ten as a small superscript above the tens column (or just remember it).

   ¹
   4 7
 + 2 8
 -----
     5

Step 2 — tens. 4 + 2 + 1 (carried) = 7. Write 7.

   ¹
   4 7
 + 2 8
 -----
   7 5

47 + 28 = 75.

Subtracting example: 63 − 28

   6 3
 − 2 8
 -----

Step 1 — ones. 3 − 8 doesn't work — 3 is too few.

• Borrow one ten from the tens column. The 6 tens become 5 (and one ten becomes 10 ones).
• On top: instead of 3 you now have 13.
• 13 − 8 = 5.

Step 2 — tens. On top you now have 5 (after borrowing), at the bottom 2. 5 − 2 = 3. 63 − 28 = 35.

Common mistakes

1. Misaligned digits. If you write 8 (a ones digit) under 2 (a tens digit), you've mixed up ones and tens. Always draw the columns — tens on the left, ones on the right. 2. Forgotten carried ten. You wrote it as a small superscript and must add it when you compute the tens. 3. Forgetting that the top has lost a ten after borrowing. After borrowing, those 6 tens are now only 5.

Summary

• Column method: ones under ones, tens under tens.
• Start with the ones, move to the tens.
• When carrying, write a tiny 1 above the tens column and don't forget to add it.
• When borrowing, subtract one ten from the top number's tens digit.