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Primary Maths · Year 4Chapter 4 · Multiplying and dividing
Year 4 · Short Multiplication and Short Division · Lesson 4.1

Year 4: Multiplying and dividing by 10 and 100

Part of the Year 4 short multiplication and short division chapter. Chapter overview · whole chapter on one page · Download worksheet (PDF) · Answers (PDF)

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4.1

Multiplying and dividing by 10 and 100

Year 4
Multiply and divide whole numbers by 10 and 100

Each column in a number is worth ten times the column on its right. Ones, tens, hundreds, thousands: every step to the left makes the value ten times bigger. That is why multiplying and dividing by 10 and 100 is so quick. You only have to move the digits.

Multiplying by 10. Take 34. It is 3 tens and 4 ones. Multiply by 10 and every digit becomes ten times bigger. 3 tens become 3 hundreds. 4 ones become 4 tens. Every digit moves one place to the left. That leaves the ones place empty, so we fill it with a 0. So 34 × 10 = 340.

OTHTh34340× 10
34 × 10 = 340. Each digit moves one place to the left. The orange 0 holds the empty ones place.

Multiplying by 100 makes every digit 100 times bigger. Every digit moves two places to the left and two zeros fill the gaps. 52 × 100 = 5,200. It is the same as multiplying by 10 and then by 10 again.

OTHTh525200× 100
52 × 100 = 5,200. Each digit moves two places to the left.

Dividing is the opposite. Dividing by 10 makes every digit ten times smaller, so each digit moves one place to the right. Dividing by 100 moves each digit two places to the right. In this chapter the numbers divide exactly, which means they end in enough zeros. 4,500 ÷ 100 = 45.

OTHTh450045÷ 100
4,500 ÷ 100 = 45. Each digit moves two places to the right. The two zeros at the end fall off.

The zero is a place holder. You may have heard “just add a zero”. That is not the real rule. The digits move, and the zero only holds the empty place so that the other digits stay in the right columns. This matters: 305 × 10 = 3,050, and when you divide, only the zeros at the end fall off: 3,050 ÷ 10 = 305.

Key words. A digit is one of the symbols 0 to 9. A place holder is a zero that keeps the other digits in the right columns. Place value is what each digit is worth.

Key ideas

  • × 10: every digit moves 1 place to the left. Put a 0 in the ones place.
  • × 100: every digit moves 2 places to the left. Put 0 in the tens and ones places.
  • ÷ 10: every digit moves 1 place to the right. (The number must end in 0.)
  • ÷ 100: every digit moves 2 places to the right. (The number must end in 00.)
  • × 100 is the same as × 10 and then × 10 again.
  • Multiplying and dividing undo each other: 34 × 10 = 340 and 340 ÷ 10 = 34.
Worked example 4.1.1
Work out 36 × 10.

Key words. A digit is one of the symbols 0 to 9. A place holder is a zero that keeps the other digits in the right columns. Place value tells you what each digit is worth.

Method. To multiply by 10, move every digit 1 place to the left and put a zero in the empty place on the right. Do not just “add a zero”: it is the digits that move.

  1. Say what each digit is worth. The digits have different values because of their columns.
    3 tens
    6 ones
  2. Make every digit 10 times bigger. Multiplying by 10 makes each digit 10 times bigger. That moves it 1 place to the left.
    3 tens become 3 hundreds
    6 ones become 6 tens
  3. Show it in a place-value table. The 1 gap on the right is empty, so we put a zero there.
    OTHTh36360× 10
  4. Write the answer. The zero place holders keep every digit in the right column.
    36 × 10 = 360

Check. Divide back: 360 ÷ 10 = 36. ✓

Watch out. Move the digits, then fill the empty place with a zero. Check the answer is 10 times bigger.

Worked example 4.1.2
Work out 47 × 100.

Key words. A digit is one of the symbols 0 to 9. A place holder is a zero that keeps the other digits in the right columns. Place value tells you what each digit is worth.

Method. To multiply by 100, move every digit 2 places to the left and put 2 zeros in the empty places on the right. Do not just “add a zero”: it is the digits that move.

  1. Say what each digit is worth. The digits have different values because of their columns.
    4 tens
    7 ones
  2. Make every digit 100 times bigger. Multiplying by 100 makes each digit 100 times bigger. That moves it 2 places to the left.
    4 tens become 4 thousands
    7 ones become 7 hundreds
  3. Show it in a place-value table. The 2 gaps on the right are empty, so we put zeros there.
    OTHTh474700× 100
  4. Write the answer. The zero place holders keep every digit in the right column.
    47 × 100 = 4,700

Check. Divide back: 4,700 ÷ 100 = 47. ✓

Watch out. Move the digits, then fill the empty places with zeros. Check the answer is 100 times bigger.

Worked example 4.1.3
Work out 520 ÷ 10.

Key words. A digit is one of the symbols 0 to 9. A place holder is a zero that keeps the other digits in the right columns. Place value tells you what each digit is worth.

Method. To divide by 10, move every digit 1 place to the right. A number that ends in 1 zero will divide exactly.

  1. Look at the end of the number. Are there at least 1 zero? Every digit moves 1 place to the right, so the ones column must be able to hold the moved digits. A number that ends in 1 zero divides exactly.
    520 ends in 1 zero. ✓
  2. Make every digit 10 times smaller. Dividing by 10 makes each digit 10 times smaller. That moves it 1 place to the right.
    5 hundreds become 5 tens
    2 tens become 2 ones
  3. Show it in a place-value table. The last 1 zero falls off the end.
    ThHTO
    520520
    ÷ 10→ 1 place right
    5252
  4. Write the answer.
    520 ÷ 10 = 52

Check. Multiply back: 52 × 10 = 520. ✓

Watch out. Move the digits 1 place to the right. Check the answer is 10 times smaller.

Worked example 4.1.4
Work out 3,400 ÷ 100.

Key words. A digit is one of the symbols 0 to 9. A place holder is a zero that keeps the other digits in the right columns. Place value tells you what each digit is worth.

Method. To divide by 100, move every digit 2 places to the right. A number that ends in 2 zeros will divide exactly.

  1. Look at the end of the number. Are there at least 2 zeros? Every digit moves 2 places to the right, so the ones column must be able to hold the moved digits. A number that ends in 2 zeros divides exactly.
    3,400 ends in 2 zeros. ✓
  2. Make every digit 100 times smaller. Dividing by 100 makes each digit 100 times smaller. That moves it 2 places to the right.
    3 thousands become 3 tens
    4 hundreds become 4 ones
  3. Show it in a place-value table. The last 2 zeros fall off the end.
    ThHTO
    3,4003400
    ÷ 100→ 2 places right
    3434
  4. Write the answer.
    3,400 ÷ 100 = 34

Check. Multiply back: 34 × 100 = 3,400. ✓

Watch out. Move the digits 2 places to the right. Check the answer is 100 times smaller.

Worked example 4.1.5
Work out 305 × 10. Take care with the zero in the middle.

Key words. A digit is one of the symbols 0 to 9. A place holder is a zero that keeps the other digits in the right columns. Place value tells you what each digit is worth.

Method. To multiply by 10, move every digit 1 place to the left and put a zero in the empty place on the right. Do not just “add a zero”: it is the digits that move.

  1. Say what each digit is worth. The digits have different values because of their columns.
    3 hundreds
    5 ones
  2. Make every digit 10 times bigger. Multiplying by 10 makes each digit 10 times bigger. That moves it 1 place to the left.
    3 hundreds become 3 thousands
    5 ones become 5 tens
  3. Show it in a place-value table. The 1 gap on the right is empty, so we put a zero there.
    ThHTO
    305305
    × 10← 1 place left
    3,0503050
  4. Write the answer. The zero place holders keep every digit in the right column.
    305 × 10 = 3,050

Check. Divide back: 3,050 ÷ 10 = 305. ✓

Watch out. Move the digits, then fill the empty place with a zero. Check the answer is 10 times bigger.

Worked example 4.1.6
Find the missing number.   45 × ? = 4,500

Key words. A digit is one of the symbols 0 to 9. A place holder is a zero that keeps the other digits in the right columns. Place value tells you what each digit is worth.

Method. Find out how far the digits move. One place to the left is × 10. Two places to the left is × 100.

  1. Compare the two numbers. 45 is on one side, 4,500 is the answer.
    45 → 4,500
  2. Count how many places the digits moved. Each digit of 45 moved to the left. Count the places.
    ThHTO
    4545
    × 100← 2 places left
    4,5004500
  3. Say what the number is. Digits that move 2 places to the left mean × 100.
    45 × 100 = 4,500

Check. 45 × 100 = 4,500. ✓

Watch out. Put your answer back in the box and read the whole sentence. It must be true.

Worked example 4.1.7
A packet holds 100 stickers. A shop has 28 packets. How many stickers does the shop have?

Key words. A digit is one of the symbols 0 to 9. A place holder is a zero that keeps the other digits in the right columns. Place value tells you what each digit is worth.

Method. To multiply by 100, move every digit 2 places to the left and put 2 zeros in the empty places on the right. Do not just “add a zero”: it is the digits that move.

  1. Choose the operation. Each packet holds 100 stickers, so we multiply the number of packets by 100.
    28 × 100
  2. Say what each digit is worth. The digits have different values because of their columns.
    2 tens
    8 ones
  3. Make every digit 100 times bigger. Multiplying by 100 makes each digit 100 times bigger. That moves it 2 places to the left.
    2 tens become 2 thousands
    8 ones become 8 hundreds
  4. Show it in a place-value table. The 2 gaps on the right are empty, so we put zeros there.
    ThHTO
    2828
    × 100← 2 places left
    2,8002800
  5. Write the answer. Include the unit.
    2,800 stickers

Check. Divide back: 2,800 ÷ 100 = 28. ✓

Watch out. Move the digits, then fill the empty places with zeros. Check the answer is 100 times bigger.

Watch out

Do not just “add a zero” or “take away a zero”. Think about the digits moving. To multiply by 100, move two places (two zeros). To divide by 10, take off one zero only: 600 ÷ 10 = 60, not 6. And a zero inside a number stays: 305 × 10 = 3,050.

Exercise 4A  Multiplying and dividing by 10 and 100

Non-calculator
1
FQuick-fire! Multiply by 10.
(a)8 × 10 =
(b)23 × 10 =
(c)65 × 10 =
(d)90 × 10 =
(e)147 × 10 =
(f)380 × 10 =
2
FQuick-fire! Multiply by 100.
(a)4 × 100 =
(b)17 × 100 =
(c)60 × 100 =
(d)205 × 100 =
(e)308 × 100 =
(f)99 × 100 =
3
FQuick-fire! Divide by 10.
(a)70 ÷ 10 =
(b)340 ÷ 10 =
(c)800 ÷ 10 =
(d)1,250 ÷ 10 =
(e)4,060 ÷ 10 =
(f)9,000 ÷ 10 =
4
FQuick-fire! Divide by 100.
(a)600 ÷ 100 =
(b)4,700 ÷ 100 =
(c)3,000 ÷ 100 =
(d)9,500 ÷ 100 =
(e)7,200 ÷ 100 =
(f)8,000 ÷ 100 =
5
FMixed quick-fire! Watch the signs.
(a)56 × 10 =
(b)300 ÷ 10 =
(c)7 × 100 =
(d)2,900 ÷ 100 =
(e)410 × 10 =
(f)6,000 ÷ 10 =
(g)83 × 100 =
(h)500 ÷ 100 =
6
FEach picture shows the digits of a number moving. Write the calculation and the answer that each picture shows.
(a)
OTHTh38380× 10
(b)
OTHTh670067÷ 100
(c)
OTHTh525200× 100
7
FFind the missing number in each sentence.
(a)45 × ? = 4,500
(b)8 × ? = 80
(c)? × 10 = 370
(d)? × 100 = 6,200
(e)? ÷ 10 = 54
(f)900 ÷ ? = 9
8
FWrite down:
(a)a number that is 10 times bigger than 76
(b)a number that is 100 times bigger than 9
(c)a number that is 10 times smaller than 830
(d)a number that is 100 times smaller than 6,100
9
PSThere are 10 pencils in a box. A school orders 85 boxes. How many pencils is that?
10
PSA sweet costs 10p. Zainab buys 64 sweets. How much does she spend? Give your answer in pence.
11
PSA rope is 3,500 cm long. There are 100 cm in 1 m. How many metres long is the rope?
12
RZainab says, “700 ÷ 10 = 7, because you take the zeros away.” Is Zainab right? Explain.
13
RAlways, sometimes or never?   When you multiply a whole number by 10, the answer ends in 0.
14
RWhich one is the odd one out?   A  5 × 100    B  50 × 10    C  500 × 10    D  5,000 ÷ 10
15
RI think of a number. I multiply it by 100 and get 6,300.
(a)What number did I think of?
(b)What do I get if I divide 6,300 by 10?

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