The big ideaWhen there are too few ones, one ten trades down for ten ones.
How it works
Look at the ones first: can you take them away? If there are enough, just take them away.
If there are too few ones, borrow a ten from the row above. One bead steps down from the tens row and brings ten ones.
Then take the rest away, and move on to the tens.
Carrying goes up and borrowing goes down. They are the same trade, run the other way round.
Borrowing, step by step.
Example 1: Zoe wants nine
43 - 9: the ones row has 3 and 9 must go. Take away the 3 you have, and 6 more must go.
Open a ten: one bead steps down and the ones row has ten ones.
Take away 6 of them: 10 - 6 = 4 ones. The tens row has 4 - 1 = 3 tens.
43 - 9 = 34
3 tens and 4 ones are left: 34.
Example 2: Open a ten
52 - 17: 2 ones is too few to take away 7, so open a ten.
The frame is now 4 tens and 12 ones.
Ones: 12 - 7 = 5. Tens: 4 - 1 = 3.
52 - 17 = 35
3 tens and 5 ones are left: 35.
Example 3: A bigger borrow
74 - 38: 4 ones is too few to take away 8, so open a ten.
The frame is now 6 tens and 14 ones.
Ones: 14 - 8 = 6. Tens: 6 - 3 = 3.
74 - 38 = 36
3 tens and 6 ones are left: 36.
Example 4: Another borrow
66 - 28: 6 ones is too few to take away 8, so open a ten.
The frame is now 5 tens and 16 ones.
Ones: 16 - 8 = 8. Tens: 5 - 2 = 3.
66 - 28 = 38
3 tens and 8 ones are left: 38.
Words to know
borrow
Trade one ten down for ten ones.
open a ten
Slide one bead down from the tens row so the ones row gets ten ones.
trade
Ten beads on one row become one bead on the next row up.
carry
Ten ones trading up for one ten. Borrowing is its mirror.
Watch out
Do not take the smaller digit from the bigger one. In 52 - 17 the ones are 2 and 7, and 7 is more than 2.
Opening a ten changes how the beads sit, not how many there are.
After you borrow, the tens row has one fewer bead. Remember it.
Try it
What is 61 - 18?
What is 83 - 47?
What is 40 - 15?
Does 68 - 45 need a borrow?
Answers: 1. 43 · 2. 36 · 3. 25 · 4. no
Try at homeAsk a grown-up to help you tie dried beans or pasta into bundles of ten with elastic bands. Make a number like forty-three. Then a customer asks for nine. You have only three loose ones, so open a bundle! Swap, and be the customer.
For grown-upsTie dried beans or pasta into bundles of ten with elastic bands. Make 43 and be the customer who asks for 9, so your child must open a bundle.
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