Adding and subtracting decimals works exactly like column addition — with one golden rule: line up the decimal points. The point keeps each digit in its proper column (ones under ones, tenths under tenths), so once the points are stacked you just add or subtract column by column as with whole numbers.

If one number has fewer decimal places, fill the gaps with zeros so the columns match. For example, to work out 3.4 + 2.7 write 3.4 as 3.40 and stack them with the points aligned:

   3.40
 + 2.70
 ------
   6.10
      

Add the hundredths, then the tenths, then the ones, carrying a ten just as before — and bring the decimal point straight down into the answer. So 3.4 + 2.7 = 6.1 (the trailing zero in 6.10 can be dropped).

Subtraction is the same idea. To work out 5.2 - 1.45, pad 5.2 to 5.20 so both numbers have two decimal places, then subtract column by column (borrowing where needed):

   5.20
 - 1.45
 ------
   3.75
      

The point in the answer sits directly under the points above it, so the result is 3.75.