Subtraction with large numbers

When larger numbers are involved, use the following procedure. Subtraction is more complicated than addition, so pen and paper are usually required.

• Start with the right-most digit of each number.
• If the answer of subtracting these digits will not be less than zero, subtract these digits. The answer is the right-most digit of the answer.
  • In this case, move one digit left in each number and repeat the process.
• If it was not possible to perform the subtraction because the result would be less than zero, add 10 to the digit of the first number to make the subtraction possible. Do the subtraction, the answer is the right-most digit of the answer.
  • Reduce the next digit to the left in the first number by 1. This will be possible if that digit is 1 or more.
    • If reducing that digit was possible, the borrowing procedure is complete. Continue to the next round, moving one digit to the left in each number. You will now use the reduced digit in the next round.
    • If reducing that digit was not possible because it was 0, add 10 to it and then reduce it by one. Now one must borrow again from next left digit. Reduce the next digit to the left by 1, borrowing again if it is 0. Continue this process until you reduce by 1 a digit that is 1 or more. Then the borrowing procedure is complete. Repeat the process with the next digit of each number. You will now be using the first of the reduced digits in the next round.

To make this clearer, see the examples below.

Example 1, no borrowing required.

Example 2

Example 3