The Big Question: To Regroup or Not?

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When adding two-digit numbers, which column do you always add first?

The ones column.

What is the 'magic number'? If the sum in the ones column is this number or more, you must regroup.

\(10\)

Let's add \(25 + 33\). Look at the ones: \(5 + 3\). Do you need to regroup?

No, because \(5 + 3 = 8\), which is less than \(10\).

Let's add \(47 + 28\). Look at the ones: \(7 + 8\). Do you need to regroup?

Yes, because \(7 + 8 = 15\), which is \(10\) or more.

Let's add \(61 + 19\). Look at the ones: \(1 + 9\). Do you need to regroup?

Yes, because \(1 + 9 = 10\).

Problem: \(32 + 54\). Do the ones digits require regrouping?

No, because \(2+4=6\).

Problem: \(18 + 22\). Do the ones digits require regrouping?

Yes, because \(8+2=10\).

Problem: \(73 + 19\). Do the ones digits require regrouping?

Yes, because \(3+9=12\).

Problem: \(55 + 25\). Do the ones digits require regrouping?

Yes, because \(5+5=10\).

Problem: \(81 + 11\). Do the ones digits require regrouping?

No, because \(1+1=2\).

What does it mean if the sum of the ones digits is \(9\) or less?

You do not need to regroup. You just write the sum in the ones place of the answer.

What does it mean if the sum of the ones digits is \(10\) or more?

You MUST regroup! This means you will carry a ten over to the tens column.