standard_description: Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.
problem_activity: Problem 1
a.   Here are two ways to find the area of a rectangle that is 23 units by 31 units.
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What do the area models have in common? How are they alike?
How are they different?
If you were to find the area of a rectangle that is 37 units by 49 units, which of the two ways of decomposing the rectangle would you use? Why?
b.   You might be familiar with different ways to write multiplication calculations. Here are two ways to calculate
$$23\times31$$
.
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In Calculation A, how are each of the partial products obtained? For instance, where does the 3 come from?
In Calculation B, how are the 23 and 690 obtained?
Look at the area models in Part (a). Which area model corresponds to Calculation A? Which one corresponds to Calculation B?
How are the partial products in Calculation A and the 23 and 690 in Calculation B related to the numbers in the area models?
Problem 2
a.   Label the area model to represent
$$37\times49$$
and to find that product.
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Decompose each number into its base-ten units (tens, ones, etc.) and write them in the boxes on each side of the rectangle.
Label regions A, B, C, and D with their areas. Show your reasoning.
Find the product that the area model represents. Show your reasoning.
b.   Here are two ways to calculate 37 × 49. Each number with a box gives the area of one or more regions in the area model.
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In the boxes next to each number, write the letter(s) of the corresponding region(s).
In Calculation B, which two numbers are being multiplied to obtain 333? Which two are being multiplied to obtain 1,480?
Problem 3
Estimate the following products. Then solve using the standard algorithm. If you get stuck, use an area model and/or the partial products algorithm to help.
a.
$$73\times84$$
b.
$$48\times67$$


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