MENTAL MATH PRACTICE

Addition

Example 1 :

40 + 30

Solution :

Both the two digit numbers have zero at one's place. So, add the digits at ten's place and take one zero along with the result. 

4 + 3 = 7

Therefore,

40 + 30 = 70

Example 2 :

80 + 50

Solution :

8 + 5 = 13

Therefore,

80 + 50 = 130

Example 3 :

46 + 30

Solution :

= 46 + 30

= 40 + 6 + 30

= (40 + 30) + 6

= 70 + 6

= 76

Example 4 :

52 + 34

Solution :

= 52 + 34

= 50 + 2 + 30 + 4

= (50 + 30) + (2 + 4)

= 80 + 6

= 86

Example 5 :

126 + 78

Solution :

= 126 + 78

= 120 + 6 + 70 + 8

= (120 + 70) + (6 + 8)

= 190 + 14

= 190 + 10 + 4

= 200 + 4

= 204

Example 6 :

208 + 126

Solution :

= 208 + 126

= 200 + 8 + 120 + 6 

= (200 + 120) + (8 + 6)

= 320 + 14

= 334

Example 7 :

85 + 49

Solution :

= 85 + 49

Here, 85 is close to the round figure 100. To make 85 as 100, we need to add 15 to 85. Take 15 from 49 and add it to 85 and do the rest of the process.

= 85 + 15 + 24

= 100 + 24

= 124

Example 8 :

291 + 76

Solution :

= 291 + 25

= 291 + 9 + 16

= 300 + 16

= 316

Subtraction

Example 9 :

70 - 30

Solution :

Both the two digit numbers have zero at one's place. So, Subtract the digits at ten's place and take one zero along with the result. 

7 - 3 = 4

Therefore,

70 - 30 = 40

Example 10 :

80 - 50

Solution :

8 - 5 = 3

Therefore,

80 - 50 = 30

Example 11 :

56 - 20

Solution :

= 56 - 20

= 50 + 6 - 20

= (50 - 20) + 6

= 30 - 6

= 24

Example 12 :

84 - 55

Solution :

= 84 - 55

= 84 - (50 + 5) 

= 84 - 50 - 5

= 34 - 5

= 29

Example 13 :

126 - 78

Solution :

= 126 - 78

= 126 - (70 + 8)

= 126 - 70 - 8

= 56 - 8

= 48

Example 14 :

208 - 126

Solution :

= 208 - 126

= 208 - (100 + 26) 

= 208 - 100 - 26

= 108 - 26

= 108 - (20 + 6)

= 108 - 20 - 6

= 88 - 6

= 82

Multiplying a Two digit Number by One Digit Number

Example 15 :

17 x 5

Solution :

= 17 x 5

= (10 + 7) x 5

It is easier to multiply 10 by any number. Multiplying 10 by 5 results 50. Multiplying 7 by 5 results 35. Adding 50 and 35, we get 75.

= 50 + 35

= 85

Example 16 :

25 x 8

Solution :

= 25 x 8

4 times 25 is equal to 100. So, we can do the above multiplication as shown below.

= 25 x 4 x 2

= 100 x 2

= 200

Example 17 :

25 x 11

Solution :

= 25 x 11

10 times 25 is equal to 250. To get the answer for 11 times 25, take 10 times 25 and add one 25 to the result.

= 250 + 25

= 275

Example 18 :

49 x 7

Solution :

= 49 x 7

Consider the rounded figure near to 49, that is 50. So, we can write 49 as (50 - 1) and multiply it by 7 as shown below. 

= (50 - 1) x 7

To multiply 50 by 7, multiply 5 by 7 and take zero along with the result. 5 times 7 is 35. Then, 50 times 7 is 350.

= 350 - 7

= 343

Example 19 :

62 x 4

Solution :

= 62 x 4

= (60 + 2) x 4

= 240 + 8

= 248

Example 20 :

99 x 9

Solution :

= 99 x 9

= (100 - 1) x 9

= 900 - 9

= 891

Multiplying a Three digit Number by One Digit Number

Example 21 :

125 x 4

Solution :

= 125 x 4

Consider the rounded figure near to 125, that is 100. So, we can write 125 as (100 + 25) and multiply it by 4 as shown below. 

= (100 + 25) x 4

= 400 + 100

= 500

Example 22 :

102 x 8

Solution :

= 102 x 8

= (100 + 2) x 8

= 800 x 16

= 816

Example 23 :

250 x 8

Solution :

= 250 x 8

= 250 x 4 x 2

To multiply 250 by 4, multiply 25 by 4 and take one zero along with the result. 25 times 4 is 100. Then, 250 times 4 is 1000.

= 1000 x 2

= 2000

Example 24 :

298 x 7

Solution :

= 298 x 7

= (300 - 2) x 7

To multiply 300 by 7, multiply 3 by 7 and take two zeros along with the result. 3 times 7 is 21. Then, 300 times 7 is 2100.

= 2100 - 14

= 2086

Multiplying a Two Digit Number by Another Two digit Number

Example 25 :

13 x 17

Solution :

= 13 x 17

Step 1 :

Choose one of the two numbers, larger one is preferred, that is 17. Add the digit in one's place in the other number.

= 17 + 3

= 20

Step 2 :

Multiply 20 by 10. And also, multiply the digits in one's place of both the numbers.

Step 3 :

Add the results of the above two multiplications.

= 200 + 21

= 221

Therefore,

13 x 17 = 221

Example 26 :

14 x 16

Solution :

Step 1 :

= 16 + 4

= 20

Step 2 :

Step 3 :

= 200 + 224

= 224

Therefore,

14 x 16 = 224

Example 27 :

17 x 15

Solution :

Step 1 :

= 17 + 5

= 22

Step 2 :

Step 3 :

= 220 + 35

= 255

Therefore,

17 x 15 = 255

Example 28 :

11 x 12

Solution :

Step 1 :

= 12 + 1

= 13

Step 2 :

Step 3 :

= 130 + 2

= 132

Therefore,

11 x 12 = 132

Example 29 :

18 x 19

Solution :

Step 1 :

= 19 + 8

= 27

Step 2 :

Step 3 :

= 272 + 72

= 342

Therefore,

18 x 19 = 342

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