Shortcut for Multiplication of Complementary Numbers

Multiplication of Complementary Numbers:
42     73  122
   x   48     x  77      x 128
      -----  -------    -----------
If you do practice this method, for Multiplication of Complementary numbers you can do within 5 seconds.

Shortcut Explanation Steps :
First check it addition of unit digits are equal to 10 and remaining numbers are same i mean tens digit place ( remaining part is same in both digits)

Step 1: Tens digit number multiply with next number of tens digit values
Step 2: Multiply unit digits then after write the value after step 1

In Example 1  (4 x 5)(2 x 8)
Answer : 2016

In Example 2  (7 x 8)(3 x 7)
Answer : 5621

In Example 3  (12 x 13)(2 x 8)
Answer : 15616

Complementary Numbers Multiplication of Two digit ends with 5 

It is similar to above method
45 x 45 = 2025
(4 x 5)(5 x 5)
  2025