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Find out how to 'borrow' from the tens place when you don't have enough ones to subtract.
Imagine you have 32 stickers and want to give 8 to your best friend. You look at your 2 loose stickers and think, 'Oh no, I don't have enough!' But wait—you have 3 full sheets of 10 stickers each. What if you could 'break' one sheet open?
When we subtract, we always start in the ones place. Sometimes, the number on the top is smaller than the number on the bottom. For example, in , we can't take away from . We call this having 'more on the floor.' When this happens, we need to go to our neighbor—the tens place—and borrow a ten. This is called regrouping or decomposing a ten.
Quick Check
In the problem , do we have 'more on the floor' in the ones place?
Answer
Yes, because 7 is bigger than 5.
Think of a ten-rod as a kit of 10 individual cubes glued together. When we regroup, we take 1 ten from the tens column and move it to the ones column. But first, we have to 'un-glue' it! That 1 ten becomes 10 ones. Now, you add those 10 ones to the ones you already had. If you had 2 ones, you now have ones. Now you have plenty to subtract from!
Let's solve : 1. Look at the ones: . We need more ones! 2. Go to the tens: Take 1 ten from the 3. Now we have 2 tens left. 3. Move the ten: Give those 10 ones to the 2. Now we have 12 ones. 4. Subtract the ones: . 5. Bring down the remaining tens: 2. 6. The answer is .
Quick Check
If you have 4 tens and you 'borrow' one for the ones place, how many tens are left?
Answer
3 tens.
Now let's try it with two-digit numbers on both sides! The steps are the same, but we have one extra step at the end: subtracting the remaining tens. Always remember to cross out the old numbers and write the new ones above them so you don't get confused. This keeps your 'math brain' organized!
Solve : 1. Ones place: . More on the floor! Borrow from the 5. 2. The 5 tens become 4 tens. The 3 ones become 13 ones. 3. Subtract ones: . 4. Subtract tens: . 5. Final Answer: .
Solve : 1. Ones place: . We definitely need to borrow! 2. Borrow 1 ten from 7. The 7 becomes 6. 3. The 0 ones become 10 ones (). 4. Subtract ones: . 5. Subtract tens: . 6. Final Answer: .
Which of these problems requires regrouping?
When we regroup, 1 ten becomes how many ones?
In the problem , after borrowing from the tens place, the 6 becomes a 5.
Review Tomorrow
Tomorrow morning, try to explain to someone why we can't just subtract in the problem .
Practice Activity
Find 5 items (like beans or blocks). Group them into tens and ones. Practice 'breaking' a group of ten to subtract a larger number of ones!