Thursday, June 28, 2018

Adding and Subtracting Decimal Numbers

When we add or subtract decimal numbers, we obtain a decimal number. What technique should we use when we cannot use a calculator?

I. The numbers are written with the same number of decimal places


We just need to place the digits of the same place values in the same columns.

For example, to calculate 3.89 + 12.03, we can set out the calculations like this:

We correctly line up the digits of the whole numbers. Then, to the right of the decimal point, we place the digits for the tenths under one another, and so on for the hundredths and the other decimals.

Then we can carry out the addition using the normal technique.


We use the same method for subtraction.


II. The numbers are not written with the same number of decimal places


It is always possible to write the decimal numbers with the same number of decimal places by adding zeros to the right of the decimal point.

Thus, 59.8 – 2.934 = 59.800 – 2.934. By laying out the numbers in the same way as before, we can find the result as follows:


Read more:
Reading and Writing Decimal Numbers
Multiplying or Dividing a Decimal Number by 10, 100, or 1,000
Multiplying Decimal Numbers
Dividing Decimal Numbers
Converting Decimals to Fractions
The Effect of Addition and Multiplication on the Order of Numbers
Describing Different Types of Numbers
Dividing Whole Numbers with a Remainder
Comparing and Ordering Decimal Numbers