Reverse addition is often one of the first ways that students learn to subtract. This refers to the fact that an addition equation can be converted to a subtraction equation by writing it backward, and vice versa. For example, the equation 5 + 2 = 7 can also be written as 7 - 2 = 5. Knowledge of reverse addition can be a powerful technique for solving subtraction equations. For example, if a student is asked to solve the equation 9 - 4 = ?, he can ask himself what number can be added to 4 to make 9. In this case, the answer is 5.
Counting back is an introductory mental math strategy that is taught to mirror the addition strategy of counting up. Counting back involves counting back from a number by ones. For example, the equation 5 -2 = ? can be solved by starting with 5 and counting back 4, 3. This strategy can be made visual by utilizing a number line and plotting the starting point as well as the numbers that will be used to count back. Counting back is most effective for young learners who are only subtracting one, two or three numbers and should not be used when the subtrahend is a number higher than four or five.
Doubles are generally easy for students to learn and involve equations such as 6 + 6 = 12 or 8 + 8 = 16. In a subtraction equation such as 16 - 8 = ?, doubles can be easily recognizable. In this case, the student should recognize that 16 is the double of 8, therefore making the answer 8. A student can build on known doubles in an equation such as 17 - 8 = ?. In this case, the student should know that 16 - 8 = 8. Then, 1 should be added to the answer to compensate for the fact that 1 needs to be added to the minuend, 16, to make 17.
Break the smaller number, or the subtrahend, into two parts to make it easier to subtract. For example, in the equation 64 - 33 = ?, the 33 can be broken down into 30 and 3. First, complete the equation 64 - 30 = ? to make a difference of 34. Then subtract the remaining 3 to make a final answer of 31.