Explain when you would not need to regroup any hundreds into thousands when multiplying a 4 digit number by a 1 digit number.

When you regroup, you rearrange groups in place value to carry out an arithmetic operation.

Regrouping is not needed when the product of individual digit does not exceed 9.

Take for instance, the following product

The above does not make use of regroup, because the result of the product of the individual digits of 1202 and 4, does not exceed 9

Take another instance:

So: 1 is written down, and 2 is regrouped with the product of 1 and 3.

Hence, regrouping would not be needed or necessary when the result of  the products of the individual digits of the 4-digit number and the 1-digit number, does not exceed 9.

Read more about regrouping at:

link

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step-by-step explanation: