How to Add and Subtract Decimals
You shouldn’t lose much sleep at night worrying about adding and subtracting decimals, because it’s nearly as easy as adding and subtracting whole numbers. Simply line up the decimal points and then add or subtract just as you would with whole numbers. The decimal point drops straight down in your answer.
To avoid mistakes (and to make your teacher happy), make sure your columns are neat. If you find it helpful, fill out the columns on the right with trailing zeros so that all numbers have the same number of decimal places. You may need to add these trailing zeros if you’re subtracting a decimal from a number that has fewer decimal places.
Sample questions

Add the following decimals: 321.81 + 24.5 + 0.006 = ?
346.316. Place the decimal numbers in a column (as you would for column addition) with the decimal points lined up:
Notice that the decimal point in the answer lines up with the others. As you can see, the columns are filled out with trailing zeros. This is optional, but do it if it helps you to see how the columns line up.
Now add as you would when adding whole numbers, carrying when necessary:

Subtract the following decimals: 978.245 – 29.03 = ?
949.215. Place the decimals one on top of the other with the decimal points lined up, dropping the decimal point straight down in the answer:
Now subtract as you would when subtracting whole numbers, borrowing when necessary:
Practice questions

Add these decimals: 17.4 + 2.18 = ?

Solve the following decimal addition: 0.0098 + 10.101 + 0.07 + 33 =

Add the following decimals: 1,000.001 + 75 + 0.03 + 800.2 = ?

Subtract these decimals: 0.748 – 0.23 = ?

Solve the following: 674.9 – 5.0001.

Find the solution to this decimal subtraction problem: 100.009 – 0.68 = ?
Following are answers to the practice questions:

17.4 + 2.18 = 19.58. Place the numbers in a column as you would for addition with whole numbers, but with the decimal points lined up. The columns are filled out with trailing zeros to help show how the columns line up:
Notice that the decimal point in the answer lines up with the others.

0.0098 + 10.101 + 0.07 + 33 = 43.1808. Line up the decimal points and do column addition:

1,000.001 + 75 + 0.03 + 800.2 = 1,875.231. Place the decimal numbers in a column, lining up the decimal points:

0.748 – 0.23 = 0.518. Place the first number on top of the second number, with the decimal points lined up. A trailing 0 was added to the second number to fill out the righthand column and emphasize how the columns line up:
Notice that the decimal point in the answer lines up with the others.

674.9 – 5.0001 = 669.8999. Place the first number on top of the second number, with the decimal points lined up. The righthand column is filled out with trailing zeros so you can complete the math:

100.009 – 0.68 = 99.329. Place the first number on top of the second number, with the decimal points lined up: