Page 71 - Start Up Mathematics_4

Solution: 13 + 5 = 18 Step 1: Add the multiplicand and the digit in the ones
place of the multiplier.

18 × 10 = 180 Step 2: Multiply the sum in step 1 by 10.
3 × 5 = 15 Step 3: Multiply the digits in the ones place of the
multiplicand and the multiplier.

180 + 15 = 195 Step 4: Add the products obtained in steps 2 and 3 to
get the product.
Thus, 13 × 15 = 195.

Simple trick

To multiply two numbers whose difference is 2, multiply the number between them by itself
and subtract 1.

Example 11: Multiply (a) 6 by 8 (b) 12 by 14.

Solution: (a) 8 – 6 = 2
The number between 6 and 8 is 7.
So, 6 × 8 = (7 × 7) – 1 = 49 – 1 = 48

(b) 12 × 14 = (13 × 13) – 1 = 169 – 1 = 168

Multiplying by 11 using addition

In your earlier class, you have already learnt a shortcut method to multiply numbers by 11.
Here is another interesting and simple method of multiplying a number by 11.

Example 12: Multiply 3,426 by 11.

Solution: 1. Write 0 on the extreme left of the multiplicand to get 03426.

2. Start from the ones place. Write 6 as the ones digit of the answer. O
6
3. Move to the tens place. The digit is 2. The digit after 2 is 6. Add 2 and 6.
2 + 6 = 8

This becomes the tens digit of the answer. T O

8 6
4. Keep moving to the left. The next digit is 4. The digit after 4 is 2.
Add 4 and 2. 4 + 2 = 6

This becomes the hundreds digit of the answer. H T O

6 8 6
5. The next digit is 3. The digit after 3 is 4. Add 3 and 4. 3 + 4 = 7

This becomes the thousands digit of the answer. Th H T O
7 6 8 6

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