Math Rules for Subtraction

Subtraction is one of the four basic arithmetic operations that follows certain rules and involves some terminology. It is a life skill that is easy to master. Subtraction is the opposite of addition. Everyone begins to learn about subtraction in the early grades of elementary school. Being good at subtraction is important because, for example, whenever you buy something, you use subtraction to figure out how much change you are due.

Identification

Subtraction compares two quantities to determine how much less one is than the other. Suppose you started with a batch of a dozen cookies. Six kids came along and each ate one cookie. You want to know how many cookies there are left. To figure this out, you must subtract six from 12. There are six cookies left. If you spend $5.25 at a restaurant and you give the clerk a $10 bill, how much change should you receive? This is another subtraction problem. $10.00 - $5.25 = $4.75.

Terminology

The parts of a subtraction problem have names. The minuend minus the subtrahend equals the remainder. For example, in the subtraction problem 17 - 3 = 14, 17 is the minuend; 3 is the subtrahend and 14 is the remainder. You could also call 14 the difference, a synonym for remainder.

Function

When you are working with positive numbers and the minuend is larger than the subtrahend the difference will be positive. If the minuend is smaller than the subtrahend and you are working with positive numbers, the difference will be negative. When the minuend equals the subtrahend, the difference is always zero. If you consider all the numbers arranged in order on a number line so that the larger numbers are to the right of the smaller numbers, subtraction involves moving towards the left starting at the minuend for as many positions until you arrive at the subtrahend. The number of positions you passed is the difference. This is true as long as you are working with positive numbers. When you are subtracting a negative number, you move towards the right on the number line starting with the minuend.

Considerations

To subtract a fraction from a fraction involves first converting the minuend and the subtrahend to fractions that share a common denominator. Suppose you want to subtract 1/2 from 3/4. 3/4 is the minuend. Convert the subtrahend, 1/2, to its equivalent fraction, 2/4. Now the problem is 3/4 - 2/4. The denominator will not change. Subtract the numerators. 3 -2 = 1. Put the new numerator over the denominator. The difference is 1/4.

Potential

Once you master the subtraction facts up to 20, you can apply that knowledge to handle complicated problems involving borrowing, decimals and mixed numbers. One of the easiest ways to remember these math facts is to organize them into math fact families in which two addition facts and two subtraction facts each share the same numbers. For example: 3 +2 = 5; 2 +3 = 5; 5 - 3 = 2; and 5 -2 =3, are related math facts. So, if you find yourself with any two of these numbers in a subtraction problem, the third number will be your answer.

Resources

• Math Help Online