Multiplication and Division
Primary 3 · Mathematics
Multiplication and Division
Lesson 2 of 3
14 min
The teacher asks the class, "What is 6 × 4?"
Gatete starts drawing dots. Six groups... four dots in each... he is still counting when Keza raises her hand and says "24".
Keza is not cleverer than Gatete. She simply knows her tables, the way you know the words of a song. Today you will learn them too.
A times table is a list of answers you keep ready in your head, so you never have to start counting from the beginning again.
You already know how to build one. To find 3 × 4 you count in fours: 4, 8, 12. A times table is just that counting, written down neatly and learned by heart.
Multiplying by 2 means doubling — taking the number twice.
2, 4, 6, 8, 10, 12, 14, 16, 18, 20.
Worked example: 7 × 2. Double 7. Seven and seven more is 14. So 7 × 2 = 14.
Count in tens: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100.
Worked example: 6 × 10 means 6 tens. Six bundles of ten sticks is sixty sticks. So 6 × 10 = 60.
Look at the answers: 3 × 10 = 30, 6 × 10 = 60, 9 × 10 = 90. The digit stays and a 0 sits behind it — because nine tens is written as 90.
Count in fives: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50.
Every answer ends in a 5 or a 0. That is a handy way to check yourself.
Worked example: 7 × 5. Count seven fives on your fingers: 5, 10, 15, 20, 25, 30, 35. So 7 × 5 = 35.
Faster way: 5 is half of 10. You know 7 × 10 = 70, so 7 × 5 must be half of 70. Half of 70 is 35. The same answer, and no counting.
Count in fours: 4, 8, 12, 16, 20, 24, 28, 32, 36, 40.
Worked example: 6 × 4. First double 6 — that is 12. Now double 12 — twelve and twelve makes 24. So 6 × 4 = 24.
That is exactly what Keza did in her head while Gatete was drawing dots.
Count in threes: 3, 6, 9, 12, 15, 18, 21, 24, 27, 30.
The 3s have no clever shortcut, so say them out loud until they sound like a song you know.
Worked example: 8 × 3. Count eight threes: 3, 6, 9, 12, 15, 18, 21, 24. So 8 × 3 = 24.
| × | 2 | 3 | 4 | 5 | 10 |
|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 10 |
| 2 | 4 | 6 | 8 | 10 | 20 |
| 3 | 6 | 9 | 12 | 15 | 30 |
| 4 | 8 | 12 | 16 | 20 | 40 |
| 5 | 10 | 15 | 20 | 25 | 50 |
| 6 | 12 | 18 | 24 | 30 | 60 |
| 7 | 14 | 21 | 28 | 35 | 70 |
| 8 | 16 | 24 | 32 | 40 | 80 |
| 9 | 18 | 27 | 36 | 45 | 90 |
| 10 | 20 | 30 | 40 | 50 | 100 |
To read it, find your number on the left and slide across to the right column. Row 7, column 4 gives 28 — and that means 7 × 4 = 28.
One mandazi costs 20 Frw. Ishimwe buys 3. How much does he pay?
20 Frw is 2 tens. Three lots of 2 tens is 6 tens, because 3 × 2 = 6. And 6 tens is 60.
So Ishimwe pays 60 Frw. Your small tables help you with big numbers too.
Tip
Build the harder tables out of the easy ones
The 4s are the 2s doubled: 7 × 2 = 14, so 7 × 4 = 28.
The 5s are the 10s halved: 8 × 10 = 80, so 8 × 5 = 40.
And if one answer slips away from you, walk to it from an answer you do remember. Stuck on 7 × 3? You know 5 × 3 = 15. Add one more 3 to get 18, and one more 3 to get 21.
Cover the big table with your hand. See how many you can do from memory.
What you learned today
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