Year 5

# Using derived facts to multiply mentally

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## Lesson details

### Key learning points

- In this lesson, we will use our knowledge of multiplying and dividing by 10, 100 and 1000 to derive facts from known facts. We will then use the distributive law and derived facts to multiply by choosing the most efficient and effective representations.

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### 5 Questions

Q1.

The strategy of separating a number into its individual units of value is commonly known as?

Approximation

Guessing

Rounding

Q2.

Half of 24 is also the same as:

240 ÷10

72 ÷ 3

96 ÷ 2

Q3.

The bar model below is used to help divide or multiply numbers. What number should be placed in the top white boxes?

12

40

6

Q4.

Which of the following number sentences will help you to solve the calculation 25 x 8?

100 x 4

250 x 80

50 x 8

Q5.

Which derived fact helps me to mentally solve 240 ÷ 5?

120 ÷ 10

120 x 10

48 ÷ 10

### 5 Questions

Q1.

Which of the following methods for answering a multiplication questions is NOT a mental strategy?

Array models

Blank number line

Marked number line

Q2.

Today we discussed distributive law in relation to multiplication. But which of the following number sentences does NOT correctly represent distributive law?

13 x 24 = 10 x 24 + 3 x 24

19 x 17 = 19 x 10 + 19 x 5 + 19 x 2

32 x 7 = 30 x 7 + 2 x 7

Q3.

Which of the following representations would be efficient for the calculation 317 x 23?

Cuisenaire Rods

Dienes

Q4.

For which of these calculations would you consider using a mental strategy and NOT a written method?

456 x 7

572 x 6

923 x 6

Q5.

Which calculation do the dienes represent?

16 x 6

603 x 6

63 x 6