One hundred flat counts as 100. One cube counts as 1. One ten long counts as 10. One hundred flat counts as 100. Remember that one cube counts as 1, one ten long counts as 10, and one hundred flat counts as 100. Also, one ten is made up of ten ones and one hundred is made up of 10 tens.

You might think that if there are no tens or ones, you can’t subtract You might think that if there are no tens or ones, you can’t subtract. In this number sentence, 700 has two zeros, so you would think you can’t take away any tens or ones. If I just take away three hundreds, because there are 3 hundreds in 342, then I get 400. But that can’t be the answer, because there is more to take away in 342. How can you take away tens and ones when there are 0s in those places?

H T O 7 0 0 - 3 4 2 Do you remember that subtraction sentence we saw at the beginning of the lesson? 700 – 342? This number sentence is a trickybecause I am taking away tens and ones, even though 700 doesn’t have any to start with. So first thing I’ll do is show 700 with base ten blocks. That means I’ll have to put 7 hundreds out, no tens, and no ones. Now when I try to take the tens and ones away, I can’t. What I’ll have to do is decompose, or break apart, one of the hundreds into ten ones. I also need ones, so I’ll decompose one of those tens into ten ones. Now that I have enough hundreds, tens, and ones to take away from, I can subtract 342 from 700. So I’ll take away 2 ones, 4 tens, and 3 hundreds. So 700 – 342 = 358.

H T O 5 7 1 – 3 2 8 Let’s look at one more subtraction sentence. First thing I do, like my other number sentences, is line up my numbers. It doesn’t matter if it’s addition or subtraction, they are really just opposite operations. I line up the numbers simply to see how many hundreds, tens, and ones I have to start with. That way I can take away the smaller number. I can lay out 571 in tens and ones blocks like this. Looking at the digits in 328, I realize that I don’t have 8 ones to take away. I know that a ten is made up of ten ones. If I had ten ones, I could take away 8 ones. What I can do now is decompose, or break apart one of the tens into ten ones like this. Now I can take away those 8 ones from 328. And then I can take away 2 tens. And I can take away 3 hundreds. So I can count everything I have left up and 571 – 328 = 233.