Multiplying any number by 1,001

Multiplying a 2-digit number by 1001

Write the 2-digit number twice and insert a ‘0’ in between.

Example: 52 × 1001 = 52052.

Multiplying numbers more than 2-digit by 1001

Rule: First, write the first three digits of the given number as the first three answer digits. Then, starting from the fourth digit of the given number, add each of the digits of the given number in turn, adding the fourth digit to the first digit, the fifth digit to the second digit, and so on. When the last digit of the original given number is reached, continue writing the remaining digits of the given number.

For example: 23,107 x 1,001 =?

The first three answer digits are 231

Starting from the fourth digit of the given number, 0, add the digits of the given number in turn.

2 + 0 = 2; 3 + 7 = 10

The 7 is the last digit of the original given number; therefore the digits of the given number not yet added, 1, 0, and 7, are merely written as the answer digits.

Whenever a sum is greater than 9, record the units digit and add the tens digit to the preceding answer digit.

So, 23,107 x 1,001 = 23,130,107