Multiplying two numbers that differ from a number by the same small number

Follow the following examples to understand the simple idea behind it.

Example 1:

99 * 101 = (100 - 1) * (100 + 1) = (100 * 100) - 1 = 10000 - 1 = 9999

Rem:

99 * 101 = (100 * 100) - (1 * 1)

99 * 101 = 100^2 - 1^2

Example 2:

997 * 1003 = (1000 - 3) * (1000 + 3) = (1000 * 1000) - (3 * 3) = 1000000 - 9 = 999991

Rem:
997 * 1003 = (1000 * 1000) - (3 * 3)

999 * 1003 = 1000^2 - 3^2

Example 3:

47 * 53 = (50 - 3) * (50 + 3) = 2500 - 9 = 2491

Rem:
47 * 53 = (50 * 50) - (3 * 3)

47 * 53 = 50^2 - 3^2 

Now, do the following exercises:

1. 195 * 205 (Rem: 195 * 205 = 200^2 - 5^2)

2. 36 * 44

3. 77 * 83

4. 148 * 152

Answers:

1. 39975

2. 1584

3. 6391

4. 22496