Multiplication of 2 two-digit numbers where the tens digit of both the numbers are same and the unit digit of the two numbers sum to 10

Example.

To calculate 66 and 64:

 

In this tens digit is same and sum of unit digits is 10
Multiply 6 by 6+1. So, 6x7 = 42. Write down 42.
Multiply together the last digits: 6*4 = 24. Write down 24.
Thus  66 x 64 = 4224.

Example.
To calculate 57 and 53:

In this tens digit is same and sum of unit digits is 10
Multiply 5 by 5+1. So, 5x6 = 30. Write down 30.
Multiply together the last digits: 7x3 = 21. Write down 21.
Thus 57 x 53 = 3021.