Multiply Multi-Digit Whole Numbers

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5th Grade Math › Multiply Multi-Digit Whole Numbers

Questions 1 - 10
1

A class is making $28$ snack bags. Each bag needs $45$ pretzels. The multiplication is $45\times 28$. Which claim about using the standard algorithm is incorrect (think about place value and partial products)?

Multiplication relies on place value because the digits in 28 represent 20 and 8.

Because 2 is in the tens place, $45\times 2$ should be added without shifting since it is still just 2.

The partial product for the 2 in 28 should represent 2 tens, so it must be written one place to the left.

You can find $45\times 28$ by adding $45\times 8$ and $45\times 20$.

Explanation

Multi-digit multiplication uses place value to correctly value each digit's contribution, such as 2 in 28 representing 20. Partial products arise from multiplying by each component, like 8 ones and 2 tens, to build the total. Aligning digits involves shifting the tens product to align with its place value during addition. This relates to the area model, where place values create grid sections whose areas sum to the product. A misconception is adding without shifting for tens, treating it as a simple digit instead of a multiple of ten. The algorithm is reliable because it decomposes and reassembles based on place values. It generalizes to ensure correct products for any multi-digit combination.

2

A student is multiplying $31\times 52$.

They correctly compute $31\times 2=62$.

For the tens digit, they compute $31\times 5=155$ but forget that the 5 represents 5 tens.

Which option is the correct product of $31\times 52$ when place value is used correctly?

1,117

1,550

1,612

217

Explanation

Multi-digit multiplication uses place value to expand digits into their true values, such as 5 in 52 meaning 50. Partial products are created by multiplying separately by each place, like 2 ones and 5 tens. Aligning digits means shifting the tens product left to represent its multiplied value accurately. This connects to the area model, with areas for each place value pair added together. A common misconception is ignoring the tens place and adding without shifting, resulting in a smaller product. The algorithm succeeds by correctly scaling and combining parts. It generalizes effectively for all multi-digit problems by adhering to place value principles.

3

Compute $58\times 42$ using the standard algorithm. Make sure the partial product for the $4$ in $42$ represents $4$ tens (so it is lined up with the tens place). What is the product?

2,356

100

1,160

2,436

Explanation

Multi-digit multiplication uses place value to decompose numbers and multiply their components separately before combining. Partial products involve multiplying the entire top number by each digit of the bottom number, respecting their places like ones or tens. Aligning digits means placing the tens partial product starting from the tens column to reflect its value. This relates to the area model, where each partial product corresponds to an area of a rectangle segmented by place values. A misconception is aligning all partial products in the ones place, which disregards the tens multiplier. The algorithm works by ensuring each partial product is scaled by its place value factor. Overall, it provides a reliable way to compute products by honoring numerical structure.

4

Use the standard algorithm for $84\times 36$. Place value matters because the $3$ in $36$ represents $3$ tens. Which step is correct for the tens partial product?

Multiply $84\times 3=252$ and write 252 starting in the tens place (so it represents 2,520).

Multiply $84\times 3=252$ and write 252 in the ones place.

Add $84+36$ to get the tens partial product.

Use only $84\times 6$ because the tens digit does not change the product.

Explanation

Multi-digit multiplication uses place value to expand numbers, treating digits as multiples of powers of ten. Partial products are formed by multiplying the whole number by each digit's value, such as 3 tens meaning a shift in placement. Aligning digits requires positioning the tens partial product to start in the tens column for accurate addition. This process mirrors the area model, with sections corresponding to ones-by-ones, ones-by-tens, and so on. A misconception is writing the tens product without shifting, which ignores the place value and leads to errors. The algorithm succeeds through systematic decomposition and recombination. It generalizes effectively because it builds on the foundational structure of our number system.

5

A school store orders $34$ boxes of pencils. Each box has $27$ pencils. Use the standard algorithm for $27\times 34$ and place value (tens and ones) to find the product. What is the product of $27\times 34$?

108

828

918

945

Explanation

Multi-digit multiplication uses place value to break down numbers into ones, tens, and higher powers of ten for accurate computation. Partial products are created by multiplying each digit of one number by the entire other number, considering their place values, such as multiplying by the ones digit first and then by the tens digit. Aligning digits involves writing the partial product for the tens digit shifted one place to the left to account for the extra factor of ten. This process connects to the area model, where rectangles represent the products of place value parts, like tens by tens or ones by tens, and their areas are summed for the total. A common misconception is forgetting to shift the partial product for the tens place, which leads to undercounting by a factor of ten. The algorithm works because it systematically accounts for every combination of place values between the two numbers. Ultimately, this ensures the product reflects the true magnitude of the multiplied quantities.

6

A student multiplies $52\times 19$ and writes these partial products:

  • $52\times 9=468$
  • $52\times 10=520$ Then the student adds them.

Which value is the correct product of $52\times 19$ (showing the partial products were combined correctly using place value)?

520

572

936

988

Explanation

Multi-digit multiplication uses place value to handle numbers by their positional values, like interpreting 1 in 19 as 10. Partial products are calculated for each place, such as multiplying by 9 ones and 1 ten separately. Aligning digits means ensuring the ten's product is shifted left to reflect its value. This connects to the area model, summing areas of rectangles defined by place value breakdowns. One misconception is treating the tens digit as just its face value without adjustment, causing misalignment. The algorithm works by accurately aggregating these adjusted products. It provides a universal method for multiplication by preserving numerical positions.

7

A student multiplies $73\times 65$ using partial products: $73\times 5=365$ and $73\times 60=4,380$. The student then combines the partial products. Which result shows the partial products combined correctly using place value?

4,415

8,760

4,380

4,745

Explanation

Multi-digit multiplication uses place value to break numbers into manageable parts like ones and tens for step-by-step multiplication. Partial products are generated by multiplying one factor by each place value component of the other, such as by 5 ones and 60 tens separately. Aligning digits ensures that when adding, the partial products line up according to their place values, like tens under tens. This connects to the area model, illustrating how each partial product fills a distinct rectangular section based on place values. A common misconception is adding partial products without considering their shifted positions, leading to incorrect totals. The algorithm is effective because it methodically combines these value-adjusted products. It generalizes to any multi-digit numbers by maintaining positional integrity.

8

A student is solving $39\times 47$ with the standard algorithm.

They correctly find $39\times 7=273$.

For the tens digit, they write $39\times 4=156$ and add it without shifting left.

Which option shows the correct product of $39\times 47$ when place value is used correctly?

1,560

1,833

429

1,113

Explanation

Multi-digit multiplication uses place value to interpret digits correctly, such as a 4 in the tens place meaning 40. Partial products come from multiplying by each digit separately, like by 7 ones and then by 4 tens, creating intermediate results. Aligning digits involves shifting the tens partial product left to account for the place value multiplier. This ties into the area model, where the total area is the sum of sub-areas representing products of place value pairs. One misconception is adding partial products without shifting, which treats tens as ones and underestimates the product. The algorithm works by correctly scaling and summing these components. It ensures precision across various number sizes by respecting place value rules.

9

Compute $67\times 24$ using the standard algorithm. Be sure to align digits by place value when adding partial products (ones under ones, tens under tens). What is the product?

1,340

268

1,608

1,708

Explanation

Multi-digit multiplication uses place value to break down and multiply numbers by their ones, tens, and other components. Partial products are the outcomes of multiplying by each digit with its place value, like 4 ones and 2 tens. Aligning digits requires positioning partial products so their place values match when adding. This aligns with the area model, representing multiplication as summed areas of place value rectangles. One misconception is forgetting to align by place, leading to incorrect column additions. The algorithm functions by methodically incorporating each place's contribution. It works universally by maintaining the integrity of the base-ten system.

10

Compute $84 \times 37$ using the standard algorithm. Which value is the correct product (and matches adding the partial products $84 \times 7$ and $84 \times 30$)?

The product is 1111.

The product is 3108.

The product is 312.

The product is 588.

Explanation

Multi-digit multiplication uses place value to break apart numbers for step-by-step multiplication. Partial products involve multiplying by each digit with its place, like 84 × 7 and 84 × 30 for 84 × 37. Digits are aligned by shifting higher place partial products to the left accordingly. This relates to the area model, where sub-areas such as 80 × 30, 80 × 7, 4 × 30, and 4 × 7 sum to 3108. A misconception is adding without shifting, which might give 588 + 252 = 840 instead of the correct total. The algorithm works because it accumulates products based on true place values. It generalizes effectively for any whole number multiplication.

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