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Ages 7+ · Abacus · Adding
Adding to make 100
tensones58
58 on the frame: five tens and eight ones.
The big ideaTo make a hundred, the ones make ten and the tens make nine.

How it works

To make a hundred, the ones make ten and the tens make nine.

Fill the ones row first, and it trades for a ten. Then fill the tens row, and it trades for one hundred.

On a number line you can see it too: hop to the next ten, then hop the rest to one hundred.

Ones make ten, tens make nine, and the carry finishes the hundred.

5060708090100+2+40starta hundred
From 58 up to 100: hop to 60, then hop 40.

Example 1: Find the missing number

  1. 58 + ? = 100. The ones row has 8, so it needs 2 more to be full.
  2. The full ones row trades for a ten, so the tens row now has 6 tens and needs 4 more.
  3. 2 + 40 = 42
  4. 58 + 42 = 100
tensones60
After the ones trade up: 6 tens and 0 ones.

Example 2: Another missing number

  1. 71 + ? = 100. The ones need 9: 1 + 9 = 10.
  2. The tens then have 7 + 1 = 8 and need 2 more: 8 + 2 = 10 tens.
  3. 9 + 20 = 29
  4. 71 + 29 = 100
tensones71
71: seven tens and one one.

Example 3: The sum

  1. Put 64 on the frame and add 36.
  2. Ones: 4 + 6 = 10, so the ones row trades for a ten.
  3. Tens: 6 + 3 + 1 = 10, so the tens row trades for a hundred.
  4. 64 + 36 = 100
hundredstensones100
One hundred is one bead on the top row.

Example 4: Check by adding

  1. 82 + ? = 100. Ones: 2 + 8 = 10. Tens: 8 + 1 + 1 = 10.
  2. So the missing number is 18.
  3. 82 + 18 = 100

Words to know

make a hundred
Add until the frame shows one bead on the top row.
trade
Ten beads on one row become one bead on the next row up.
carry
The ten that moves up a row when a row is full.
hundred
Ten tens, one bead on the top row.

Watch out

Try it

  1. 45 + ? = 100
  2. 33 + ? = 100
  3. 27 + ? = 100
  4. 19 + ? = 100
Answers: 1. 55 · 2. 67 · 3. 73 · 4. 81
Try at homeAsk a grown-up to put some coins on the table, between twenty and ninety. Say how many, then count out how many more make a hundred. First make the ones into a ten, then the tens, and check it together.
For grown-upsPut some coins on the table, between twenty and ninety, and ask how many more make a hundred. Check it together by counting up.
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