Topic D: Dividing by Two and Three Digit Divisors

Wendy Tagami; Liz Girard

Topic D: Dividing by Two and Three Digit Divisors

Finding Trial Quotients:

When dividing by 2-digit numbers, you will need to estimate the quotient. This guess is called a trial quotient.

Example A

$624 \div 24$

Rewrite as $24624$

Step 1: Divide

Think: $26$ is $3$ . So $2462$ is about $3$ .

Step 2: Multiply and subtract.

$\begin{array}{r} 3 \\ 24624 \\ 72 \end{array}$
Since 72 > 62, 3 is too large.

Step 3: Try a smaller number, multiply and subtract.

$\begin{array}{r} 2 \\ 24624 \\ 48 \\ 14 \end{array}$
Since 4 < 24, 2 is correct.

Step 4: Finish the problem.

Example B

$630 \div 15$

Rewrite as $15630$

Step 1: Divide

15 rounds to 20. Think: $26$ is $3$ . So $1563$ is about $3$ .

Step 2: Multiply and subtract.

$\begin{array}{r} 3 \\ 15630 \\ 45 \\ 18 \end{array}$
Since 18 > 15, 3 is too small.

Step 3: Try a larger number, multiply and subtract.

$\begin{array}{r} 4 \\ 24630 \\ 60 \\ 3 \end{array}$
Since 3 < 15, 4 is correct.

Step 4: Finish the problem.

Exercise 1

In each question, the trial quotient is either too large or too small. Multiply. Write too large or too small on the line. Then, write the correct trial quotient beside. Check your work using the answer key at the end of the exercise.

$\begin{array}{r} 2 \\ 25475 \end{array}$
$\begin{array}{r} 3 \\ 15682 \end{array}$
$\begin{array}{r} 5 \\ 18813 \end{array}$
$\begin{array}{r} 2 \\ 25810 \end{array}$
$\begin{array}{r} 3 \\ 33891 \end{array}$
$\begin{array}{r} 3 \\ 18819 \end{array}$
$\begin{array}{r} 3 \\ 27727 \end{array}$
$\begin{array}{r} 2 \\ 35652 \end{array}$
$\begin{array}{r} 3 \\ 25650 \end{array}$
$\begin{array}{r} 4 \\ 34176 \end{array}$
$\begin{array}{r} 4 \\ 12420 \end{array}$
$\begin{array}{r} 2 \\ 43801 \end{array}$
$\begin{array}{r} 3 \\ 31899 \end{array}$
$\begin{array}{r} 4 \\ 18648 \end{array}$
$\begin{array}{r} 4 \\ 27946 \end{array}$
$\begin{array}{r} 3 \\ 23943 \end{array}$
$\begin{array}{r} 3 \\ 24578 \end{array}$
$\begin{array}{r} 2 \\ 29406 \end{array}$
$\begin{array}{r} 2 \\ 48892 \end{array}$
$\begin{array}{r} 2 \\ 28534 \end{array}$
$\begin{array}{r} 3 \\ 37939 \end{array}$
$\begin{array}{r} 2 \\ 28854 \end{array}$

Answers to Exercise 1

too large, 1
too small, 4
too large, 4
too small, 3
too large, 2
too small, 4
too large, 2
too large, 1
too large, 2
too small, 5
too large, 3
too large, 1
too large, 2
too large, 3
too large, 3
too small, 4
too large, 2
too large, 1
too large, 1
too large, 1
too large, 2
too small, 3

Example C

$782706$

Since 78 rounds to 80, think $827$ . 8 goes into 27 ≈ 3. 3 would be a good trial quotient.

$\begin{array}{r} 3 \\ 782706 \\ 234 \\ 36 \end{array}$

Since 36 < 78, 3 is a good trial quotient.

Example D

$272205$

Since 27 rounds to 30, think $322$ . 3 goes into 22 ≈ 7. 7 would be a good trial quotient.

$\begin{array}{r} 7 \\ 272205 \\ 189 \\ 31 \end{array}$

Since 31 > 27, so 7 is too small. A better trial quotient would be 8.

Exercise 2

Find the first digit in the trial quotient. Check your work using the answer key at the end of the exercise.

$431772$ $\begin{array}{r} 4 \\ 417 \\ 16 \\ 1 \end{array}$ . 1 < 4
$643276$
$286008$
$332731$
$594164$
$752420$
$543316$
$382759$
$46387$
$353316$
$837237$
$776763$
$933724$
$524690$
$862089$
$261417$
$721462$
$276939$
$327840$
$247605$
$168640$
$453060$
$382158$
$421491$

Answers to Exercise 2

4
5
2
8
7
3
6
7
8
9
8
8
4
9
2
5
2
2
2
3
5
6
5
3

Two Digit Divisors

Dividing by large divisors is a challenge!

You must estimate how many times one number will divide into another. Use pencil and have an eraser close by when you do these questions. You will use the same steps that you already know.

Example E

$964 \div 75 =$

Step 1: Divide

Does 75 go into 9? NO
Does 75 go into 96? YES
Estimate
1. Round 75 to 80 – think “8”
2. Round 96 to 100 – think “10”
3. How many 8’s in 10? Just 1.
4. The estimate for the first digit in the trial quotient is 1.
Write 1 in the quotient above the 6 tens.

$\begin{array}{r} 1 \\ 75964 \end{array}$

Step 2: Multiply

$1 \times 75 = 75$

Write 75 under 96.

$\begin{array}{r} 1 \\ 75964 \\ 75 \end{array}$

Step 3: Subtract

$96 - 75 = 21$

Check 21 < 75? YES!

$\begin{array}{r} 1 \\ 75964 \\ 75 \\ 21 \end{array}$

Step 4: Bring down the next digit from the dividend. Now divide 214.

$\begin{array}{r} 1 \\ 75964 \\ 75 ↓ \\ 214 \end{array}$

REPEAT

Step 1: Divide

Does 75 go into 214? YES
Estimate 75 as 80 – think “8”
1. Estimate 214 as 200 – think “20”
2. 8 goes into 20 2 times ( 8 2 = 16, so 20 8 2)
The estimate for the second digit in the trial quotient is 2. Write 2 in the quotient above the 4 in the dividend.

$\begin{array}{r} 12 \\ 75964 \\ 75 ↓ \\ 214 \end{array}$

Step 2: Multiply

$2 \times 75 = 150$

Write 150 under the 214.

$\begin{array}{r} 12 \\ 75964 \\ 75 ↓ \\ 214 \\ 150 \end{array}$

Step 3: Subtract and check that the remainder is less than the divisor.

$\begin{array}{r} 12 \\ 75964 \\ 75 ↓ \\ 214 \\ 150 \\ 64 \end{array}$

Step 4: Bring down – no more digits in dividend.

To check your answer

$\begin{array}{rrr} 75 \\ \times & 12 \\ 150 \\ 750 \\ 900 \\ + & 64 \\ 964 \end{array}$

Exercise 3

Carefully divide these questions. Be careful to keep the hundreds in line with the hundreds, the tens with the tens, and so on. You might want to use squared paper for long division.

If you are having any difficulty, ask your instructor to watch you doing a few questions to be sure you are using a correct method.

$10720$
$12564$
$21882$
$22946$
$321632$
$23943$
$622528$
$712414$
$247578$
$822958$
$186250$
$251550$
$199595$
$473854$
$586500$
$249648$
$491312$
$677683$

Answers to Exercise 3

72
47
42
43
51
41
40 R48
34
315 R18
36 R6
347 R4
62
505
82
112 R4
402
26 R38
114 R45

If the estimate for your trial quotient is too large the result of the multiplication will be larger than the numbers in the dividend.

Divide: Trial estimate is 4.
Multiply:
1. $4 \times 23 = 92$
2. 92 is larger than 78, so 4 is too large an estimate. Erase it. Try 3. $3 \times 23 = 69$
3. 3 is the correct estimate and you can complete the division.

1.

$23784$

2.

\begin{array}{r} 4 \\ 23784 \\ 92 \end{array}

3.

\begin{array}{l} 34 R 2 \\ 23784 \\ 69 ↓ \\ 94 \\ 92 \\ 2 \end{array}

If the estimate is too small, the result of the subtraction will be larger than the divisor.

Divide: Trial estimate is 5.
Multiply: $5 \times 72 = 360$
Subtract:
1. $448 - 72 = 360$
2. Check 88 < 72? NO, 88 is greater than 72.
3. So 5 is too small. Erase it and use a larger number. 6 will be a better estimate.
Divide: $448 \div 82 \approx 6$
Multiply: $6 \times 72 = 432$
Subtract:
1. $448 - 432 = 16$
2. Check 16<72? YES!
Bring down the next digit and complete the division.

1.

724487

2.

\begin{array}{r} 5 \\ 724487 \\ 360 \\ 88 \end{array}

3.

\begin{array}{l} 62 R 23 \\ 724487 \\ 432 ↓ \\ 167 \\ 144 \\ 23 \end{array}

Exercise 4

Divide and check your work by multiplying. Check your work using the answer key at the end of the exercise.

$18648$
$266766$
$521968$
$848640$
$722883$
$948126$
$204060$
$471728$
$331886$
$255750$
$792765$
$428442$
$579144$
$9620160$
$7523550$

Answers to Exercise 4

36
260 R6
37 R44
102 R72
40 R3
86 R42
203
36 R36
57 R5
230
35
201
160 R24
210
314

Dividing by 10, 100, 1000…

What is the pattern? When you divide by 10

The ones digit in the dividend becomes the remainder.

$10324 = 32 R 4$

The other numbers in the dividend stay the same but each digit is one place value less.

- the hundreds become tens
- the tens become ones
- the ones become the remainder

Exercise 5

Find the quotients. Look for the pattern. Check your work using the answer key at the end of the exercise.

$1046$
$1075$
$10136$
$10832$
$10674$
$10952$
$102457$
$103685$

Answers to Exercise 5

4 R6
7 R5
13 R6
83 R2
67 R4
95 R2
245 R7
368 R5

When you divide by 100

The ones and tens digits in the dividend become the remainder.
The other digits in the dividend stay the same but each digit is two places less.
- the thousands become tens

- the hundreds become ones
- the tens and ones become the remainder

Exercise 6

Find these quotients. Look for the pattern when you divide. Check your work using the answer key at the end of the exercise

$100386$
$100995$
$100269$
$100175$
$1002948$
$1004671$
$10092045$
$10043821$

Answers to Exercise 6

3 R86
9 R95
2 R69
1 R75
29 R48
46 R71
920 R45
438 R21

When you divide by 1000

The ones, tens, and hundreds digits become the remainder.
The other digits stay the same but are three place values less.
- thousands become ones
- ten thousands become tens
- hundred thousands become hundreds

Exercise 7

Try these. Check your work using the answer key at the end of the exercise.

$10002398$
$10006475$
$10004835$
$100063291$
$100082405$
$1000293591$

Answers to Exercise 7

2 R398
6 R475
4 R835
63 R291
82 R405
293 R591

Three Digit Divisors

If the divisor has three digits, use the method you know for two-digit divisors, but estimate the divisor to the nearest hundred to find the trial quotient. Be very careful to put the first digit in the quotient in the correct place.

Example F

17902 ÷ 381 =

Step 1: Divide

Does 381 go into 1? NO
Does 381 go into 17? NO
Does 381 go into 179? NO
Does 381 go into 1790? YES
Estimate 381 as 400. Think 4.
Estimate 1790 as 1800. Think 18.
4 goes into 18 ≈ 4 times ( $4 \times 4 = 16$ )
Your estimate is 4. Write 4 in the quotient above the 0 in the dividend.

$\begin{array}{r} 4 \\ 38117902 \end{array}$

Step 2: Multiply

4 × 381 = 1524

$\begin{array}{r} 4 \\ 38117902 \\ 1524 \end{array}$

Step 3: Subtract

1790 − 1524 = 266
Check 266 < 381? YES!

$\begin{array}{r} 4 \\ 38117902 \\ 1524 \\ 266 \end{array}$

Step 4: Bring down the 2. 2662 is now the number to be divided

$\begin{array}{r} 4 \\ 38117902 \\ 1524 ↓ \\ 2662 \end{array}$

REPEAT

Step 1: Divide $2662 \div 381 =$

Estimate 381 as 400. Think of 4.
Estimate 2662 as 2700. Think 27.
4 goes into 27 ≈ 6 times ( $4 \times 6 = 24$ ).
Place this estimate in the quotient above the 2

$\begin{array}{r} 4 \\ 38117902 \\ 1524 ↓ \\ 2662 \end{array}$

Step 2: Multiply

6 × 381 = 2286[/latex]

$\begin{array}{r} 46 \\ 38117902 \\ 1524 ↓ \\ 2662 \\ 2286 \end{array}$

Step 3: Subtract

2662 − 2286 = 376
Check 376 < 381? YES!

$\begin{array}{r} 46 \\ 38117902 \\ 1524 ↓ \\ 2662 \\ 2286 \\ 376 \end{array}$

Step 4: No more digits to bring down.

17902 ÷ 381 = 46 R376

Exercise 8

Divide and check your answers. These questions are hard work! Check your work using the answer key at the end of the exercise.

$1158682$
$2052384$
$32566321$
$24113284$
$860262412$
$659270190$

Answers to Exercise 8

75 R57
11 R129
204 R21
55 R29
305 R112
410

Topic D: Self-Test

Mark /12 Aim 10/12

Divide and check your work for questions b and f using multiplication. (12 marks)

$185 \div 10 =$
$492$
$408 \div 50 =$
$726768$
$675963$
$534856$
$918736$
$265133624$
$60626094$
$100083652$

Answers to Topic D Self-Test

Divide and check your work for questions b and f using multiplication.

18 R5
23
8 R8
94
89
91 R33
96
504 R64
43 R36
83 R652

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