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Special case of Multiplication:

When the sum of unit digit place of both numbers is 10 and other digits are the same:

i) 73 × 77

Step 1:  3 × 7 = 21  [multiplication of unit digit] 

Step 2: 7 × (7 + 1) = 7 × 8 = 56 

Result is 5621

ii) 113 × 117

Step 1:  3 × 7 = 21  [multiplication of unit digit] 

Step 2: 11 × (11 + 1) = 11 × 12 = 132 

Result is 1321

iii) 59 × 51

Step 1: 9 × 1 = 09  [we need to make this 2 digit]

Step 2: 5 × (5 + 1) = 5 × 6 = 30

Result is 3009 

iv) 121 × 129

Step 1: 9 × 1 = 09 

Step 2: 12 × (12 + 1) = 12 × 13 = 156 

Result is 15609