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Multiplication of a 2-Digit Number by a 1-Digit Number Mentally
We can multiply the given number by a single digit multiplier mentally in different ways.

Doubling and Adding to Multiply by 3 and 4

• To multiply a number by 3, we first double the number and then add one more set
of the number to it.
• To multiply a number by 4, double the multiplicand and double the sum obtained to
get the answer.

Example 11: Multiply 21 by 3. Example 12: Multiply 18 by 4.

Solution: Double 21 to get Solution: Double 18 to get
21 + 21 = 42. 18 + 18 = 36
Add one more 21 to it, Double the sum to get

42 + 21 = 63 36 + 36 = 72

So, 21 × 3 = 63. So, 18 × 4 = 72.

Using Expanded Notation

Example 13: Find 47 × 8.
Solution: Write 47 in the expanded form, 47 = 40 + 7.

Multiply 40 and 7 by 8 separately.
40 × 8 = 320 7 × 8 = 56

Add the two products to get the answer.
320 + 56 = 376

So, 47 × 8 = 376.

Estimating Products
By using the method of estimation while multiplying numbers, we can find a product
quickly. To estimate the product, round off the multiplicand only to the nearest 10, 100,
etc., and then multiply.

Example 14: Multiply 47 by 2 using the estimation method by rounding off to the nearest
10 and compare with the actual value.

Solution: Estimated Value Actual Value

H T O T O
1
5 0 (47 is rounded off to 50) 4 7
× 2 × 2

1 0 0 9 4

So, the estimated value 100 is very close to the actual value 94.

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