Using the dry-erase board, or blackboard, in the front of your classroom, show your students how to write a subtraction equation. If the problem is in a word problem format, instruct a student to read the word problem and pick out the important pieces of information. Write the equation on the board, large and clear enough for your students to see in the back row. For instance, if the word problem states, “Tom has 23 pieces of candy and gives eight to Jill,” explain that the numbers of candy, and the act of giving, are important to the equation. Write “23 – 8” on the board.
Set up the problem in the right format, placing the first number on top and the second number below it. Point out that the second number lines up with the first number based on the right most digit. As an example, the “8” should line up directly under the “3” in “23.” Draw a line under the lowest number and a subtraction sign ( – ) to the left of the lower number.
Use a piece of paper to cover all of the digits in the equation except for the last one in each number. Ask your class if they can subtract these numbers and receive a positive number. If not, explain how to borrow a number from the previous digit in the top number. For instance, the subtraction problem “3 – 8” would yield a negative number, so you will need to borrow from the “2” digit.
Use a different colored marker and cross out the “2” digit, writing a “1” above it to show your class that you are borrowing a one from it. Cross out the “3” digit and write a “13” over it. Repeat your question, asking your students if they can solve the new subtraction problem and receive a positive value. As an example, the problem now shows “13 – 8,” which becomes a “5.” Write the “5” below the “8” on the board.
Repeat this process for each number until you have the solution to your equation. Demonstrate this technique to your class using other numbers, including numbers where you don't need to borrow digits to show the different ways to use these principles.