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Strategies

Practice

MENTAL CALCULATION STRATEGIES

9a x 9b
A strategy to multiply any two numbers between 90 and 99.

Example 1


98 x 97 = 9506
How we got 9506

Step 1: Subtract each number from 100
100 - 98 =   2

100 - 97 =   3
Step 2: Do cross subtraction as indicated below
98 -   3 = 95 or

97 -   2 = 95
Either way, you end up with the same answer. This forms the first part of our answer: 95

Step 3: Finally multiply the two answers from step 1 to get the second part of our answer: 06
  2 x   3 = 06
Note that we add an extra zero if the product is less than 10. Here the answer is 9506 and not 956.

Example 2


95 x 98 = 9310
How we got 9310

Step 1: Subtract each number from 100
100 - 95 =   5

100 - 98 =   2
Step 2: Do cross subtraction as indicated below
95 -   2 = 93 or

98 -   5 = 93
Either way, you end up with the same answer. This forms the first part of our answer: 93

Step 3: Finally multiply the two answers from step 1 to get the second part of our answer: 10
  5 x   2 = 10

Example 3


94 x 90 = 8460
How we got 8460

Step 1: Subtract each number from 100
100 - 94 =   6

100 - 90 = 10
Step 2: Do cross subtraction as indicated below
94 - 10 = 84 or

90 -   6 = 84
Either way, you end up with the same answer. This forms the first part of our answer: 84

Step 3: Finally multiply the two answers from step 1 to get the second part of our answer: 60
  6 x 10 = 60

Example 4


99 x 99 = 9801
How we got 9801

Step 1: Subtract each number from 100
100 - 99 =   1

100 - 99 =   1
Step 2: Do cross subtraction as indicated below
99 -   1 = 98 or

99 -   1 = 98
Either way, you end up with the same answer. This forms the first part of our answer: 98

Step 3: Finally multiply the two answers from step 1 to get the second part of our answer: 01
  1 x   1 = 01
Note that we add an extra zero if the product is less than 10. Here the answer is 9801 and not 981.

Example 5


93 x 92 = 8556
How we got 8556

Step 1: Subtract each number from 100
100 - 93 =   7

100 - 92 =   8
Step 2: Do cross subtraction as indicated below
93 -   8 = 85 or

92 -   7 = 85
Either way, you end up with the same answer. This forms the first part of our answer: 85

Step 3: Finally multiply the two answers from step 1 to get the second part of our answer: 56
  7 x   8 = 56

Example 6


96 x 95 = 9120
How we got 9120

Step 1: Subtract each number from 100
100 - 96 =   4

100 - 95 =   5
Step 2: Do cross subtraction as indicated below
96 -   5 = 91 or

95 -   4 = 91
Either way, you end up with the same answer. This forms the first part of our answer: 91

Step 3: Finally multiply the two answers from step 1 to get the second part of our answer: 20
  4 x   5 = 20