Counting on subtraction which method

Starting in the units column on the right, we need to take 9 away from 1. In our subtraction calculations, the rule as in the example above is that we never take a larger number away from a smaller number because it would give us a negative answer. In order to make the calculation work we need to ' borrow ' a number from the next column on the left. The tens column has a 0 in it so there is nothing to borrow, so we have to move to the next column to the left. The thousands column has 1, so we can borrow this and move it over to the next column on the right, the hundreds column.

We cross through the 1 in the thousands column to avoid mistakes later. One thousand is the same as 10 hundreds, so now we have 10 in the hundreds column where before we had zero:. Now that we have 10 hundreds, we can borrow one of these for the tens column. One hundred is the same as 10 tens, so we carry 10 across to the tens column. We must not forget to adjust the hundreds column, so we cross through the 10 and write 9 instead.

Finally, we can perform our subtraction in the units column by borrowing 1 ten from the tens column. Having borrowed multiple times we have arrived at our answer of 2.

If we were doing a simple addition calculation, we might count up in our heads or perhaps on our fingers. When we are doing subtraction, especially if it involves negative numbers, it helps to imagine ourselves walking along a line. Each step is a number on that line. If we start at zero, each step forward adds a number, each step backwards takes one away.

The most important thing to remember is that we always face the positive direction. You might find it helpful to think of your line as climbing up and down a ladder, with each rung being a number. Or perhaps you are more familiar with travelling up and down a high-rise block in a lift, where zero is the ground floor, positive numbers are above ground and negative numbers are in the basement.

If we were to draw that line on a piece of paper, it would look like a ruler. We can move our pen backwards and forwards along the line in the same way as imagining our steps backwards and forwards. This is called a number line , and is a very useful tool for addition and subtraction. Using our analogy, starting at zero, if we walk 19 steps forwards along the line, then 19 steps backwards, we end up back at zero.

Using our analogy, we start at zero and walk 15 steps backwards. Using our number line to help us understand, we start by standing at — We walk backwards six steps, still facing in the positive direction. We end up 21 steps backwards from zero, i. But what happens if we need to subtract a negative number from any other number?

The rule is two negatives make a positive , i. But we have a negative number to subtract, so to illustrate this we must turn around. Then we move backwards 6 places to arrive at our answer. By turning around and then moving backwards two negatives , our overall direction of travel is in a positive direction, i. However, it is very important when it comes to mathematical concepts such as vectors.

A vector has direction as well as magnitude , so for example, it is not just important how far a boat has sailed, but we also need to know the direction in which it has travelled. Add on the remainder in the larger number. In this example, you still have 27 left over once you've reached Therefore, you would add 27 to the column containing , 30 and 2. Add the numbers from your column for your final answer. In this example, you would add 27, , 30 and 2 to get Charlotte Johnson is a musician, teacher and writer with a master's degree in education.

She has contributed to a variety of websites, specializing in health, education, the arts, home and garden, animals and parenting. Write out your subtraction problem. For example, you may have - Basic Mathematics Skills. How to Learn Math With an Abacus. The Scaffold Method of Long Division. How to Calculate Binary Numbers. How to Write Numbers in Expanded Form. How to Do Long Division Math. How to Round to the Nearest Ten Thousand. How to Divide a Three Digit Number.