For 305 − 128 there are no tens at all, and only 5 ones.
Step 1: Let's Learn
Read it, or press Listen and follow the words.
You cannot open what is not there
There are no tens to break, so you must break a hundred first.
Step one
Break a hundred into ten tens. Now 2 hundreds, 10 tens, 5 ones.
Step two
Now break one of those tens into ten ones. Now 2 hundreds, 9 tens, 15 ones.
2 hundreds, 9 tens and 15 ones is still 305. Then subtract as usual.
Why a zero makes it harder
In 502 - 247 the ones need more, so you look to the tens — and the tens are 0. There is nothing there to open. You have to go one further left, open a hundred into ten tens first, and then open one of those tens into ten ones.
A worked case: 502 - 247
Open a hundred: 502 becomes 4 hundreds, 10 tens, 2 ones. Now open a ten: 4 hundreds, 9 tens, 12 ones. Ones: 12 - 7 = 5. Tens: 9 - 4 = 5. Hundreds: 4 - 2 = 2. Answer: 255. Check: 255 + 247 = 502.
Go slowly and write every step
This is the hardest procedure in Grade 2 arithmetic and speed is not the goal. Two separate openings, both written down, both done before any subtracting starts. Children who try to do it in their heads get it wrong; children who write it out get it right.
Step 2: Try It Yourself
Break the hundred first, then the ten. Two moves, in that order.
Start with
Taken away
What is left
Hundreds
1
Tens
7
Ones
7
305 − 128 = 177
Take 128 away from 305. The faded blocks are the ones that left.
Step 3: Watch an Example
One step at a time.
Watch Leo Cross the Zero
Leo needs to take 128 from 305.
- Step 1
He checks the ones: 5, and 8 have to go. Not enough.
Step 4: Your Turn
Practice makes it stick.
The Empty Column
Problem 1 of 2
2 hundreds, 9 tens and 15 ones is which number?
The Shop Again
Problem 2 of 2
A shop had 400 items and sold 156. How many are left?
items
Across the Zero
Break the hundred first, then the ten.
1 of 3
502 − 147 = ?
2 of 3
600 − 235 = ?
3 of 3
703 − 158 = ?
Step 5: Quick Check
Show what you know.
Question 1 of 2
405 − 137 = ?
Question 2 of 2
Why break the hundred before the ten?
What You Learned
- A zero in the tens means you must break twice.
- Break the hundred first, then one of the new tens.
- The amount never changes, only the grouping.