Lesson · Grade 5 · Mathematics
Multiply by 4 and by 8 by doubling
- Grade 5
- Mathematics
- 1 min 11 s
- British Columbia
- BC5-MA-19
Multiplying by 4 means doubling twice, and multiplying by 8 means doubling three times, because 2 × 2 = 4 and 2 × 2 × 2 = 8. The video works out 7 × 8 by doubling one row of 7 dots to 2, 4 and then 8 rows (7, 14, 28, 56), doubles 28 as 40 + 16 = 56, and ends on the one-line recap.
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In one sentence
The question this lesson answers
What is 7 times 8?
Doubling a number means multiplying it by 2.
Transcript
What the lesson says
What is 7 times 8?
Doubling a number means multiplying it by 2.
Double twice to multiply by 4, since 2 times 2 is 4.
Double three times to multiply by 8, since 2 times 2 times 2 is 8.
Start with one row of 7, so 7 times 1 is 7.
Double it: two rows of 7 make 14.
Double again: four rows of 7 make 28.
Double once more: eight rows of 7 is 28 plus 28.
20 plus 20 is 40, 8 plus 8 is 16, and 40 plus 16 is 56.
So 7 times 8 is 56.
One chain of doubles gave us 7 times 2, 7 times 4 and 7 times 8.
To multiply by 4, double twice, and to multiply by 8, double three times.
Curriculum
Where this fits
- British Columbia, Grade 5
Mathematics, Number: multiplication and division facts to 100 (emerging computational fluency)BC5-MA-19
Previous: Multiply by 5: Times 10, Then HalveNext: Break a number apart to multiply
- British Columbia, Grade 6
Mathematics, Number: multiplication and division facts to 100 (developing computational fluency)BC6-MA-03
Previous: Which is bigger, 0.5 or 0.45?Next: Break a number apart to multiply
- British Columbia, Grade 7
Mathematics, Number: multiplication and division facts to 100BC7-MA-02
Previous: Multiply by 5: Times 10, Then HalveNext: Multiply by 9: Times 10, Then Take One Group Away
This lesson is also listed in Grade 6 and Grade 7.
The outcomes and strands follow the British Columbia curriculum.
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