Before we finish the chapter, I want to show you what happens when we have to ungroup twice before even starting to subtract! First, picture this: let’s say we need 87 googly eyes, but there aren’t any individual boxes left—just 5 packages to open. Let’s do 500 − 87.

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For the ones column, we can’t subtract 0 − 7, right? So we try to take 1 ten from the tens column—but wait, there’s only a 0—there are no tens to take! That means we have to take 1 hundred so the 5 becomes a 4, and then that 1 hundred becomes 10 tens in the tens column. Yep, now we have 4 hundreds packages and 10 individual boxes.

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Hmm, but we still have to open one of those boxes to get to the googly eyes, right? So we have to ungroup again—this time in the tens column. So we open a box to get 10 ones, and this turns the 10 into a 9, and in the ones column, the 0 becomes 10. Finally, some ones!

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I told you it was crazy—just look at all that opening of googly eye boxes, and the subtraction hasn’t even happened yet! Now we can subtract—starting with the ones column, as always:

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So we got 500 − 87 = 413. And of course, we can check our answer by adding 413 + 87 and hoping we get 500:

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Great, we did it!

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